To have nothing further to do with them?

I think you do just have to accept they are indeed a bit shit and focus on raising your child so they don’t need these relationships.

Hopefully you’ll feel a bit liberated once the sting is out of it all. Your husband should have your back too more than he does. Sadly I think a lot of men are like this, keep quiet to keep the peace.

Your autism isn’t a problem. It’s more likely to be the massive chip you have on your shoulder. You also have unrealistic views on how families should operate

It sounds like this relationship is something you've become hyper focused on, and I understand where that comes from but their life doesn't look like yours. If they're working, their child has hobbies/school/friends and they're 45 minutes away it's not simple for you and your family to feature heavily in their every day life. You're not 10 minutes away where they can pop in on the way home for half an hour, it doesn't mean they don't love you just that they have a busy life.
If your child doesn't know anyone else at that party it's taking her cousin away from her friends to ensure your daughter isn't left out which might limit her enjoyment of her own party, can you see why that would be unfair on the child who's birthday it is? It changes the dynamic of the party.

Ultimately what I take from everything you've said is you're terrified your child will have no friends as that's been your experience, which is understandable. Being upset won't help your child's confidence though, and it sounds like this is what you should focus on to let her have healthy friendships and relationships with others.

You can’t force friendships. Even if they are cousins.
My DH has a much closer relationship with my best friends kids then he does with his cousins.

You’re putting a lot of pressure on this relationship when they clearly don’t feel the same. You need to accept that it’s okay for them to feel this way.

I know you won't like to hear this but maybe they don't like your daughter.

I don't like one of my nieces and don't want my children around her, yes that means we've drifted from some of the family.

You shouldn't force things just because they make you happy. What if your in laws are happier when their child is away from their cousin because their behavior is better? Life is super tough, and you need to prioritize that within your control

You are being unfair to your SIL and niece because your expectations are rigid and excessive.

You wanted her to be your sister when you got married. She isn't. She's your husband's sister and she isn't there to fill a gap in your life.

You want the two cousins to be friends. It's nice that they get on when they are together but your niece isn't a social band-aid for your daughter. She has lots of friends herself and is at an age where birthday parties are for pals, not merely cake with relatives.

It wasn't a "secret birthday party" that your niece had, it was a normal one. That they had to try and hide it from you speaks volumes about how they knew you'd react. Which you then did.

Your daughter shouldn't be in tears about not going to the party, that is all about how you and she are framing things. You seem to think she's entitled to be at a party where she will know no one but her cousin, and that it's cruel to exclude her. That doesn't help you daughter at all.

"Olivia had a birthday party with her school friends and is coming to celebrate with us on Sunday."

^ see? Job done.

My brother always invited our children to his family's birthday parties and after about age 5 they didn't enjoy it. They couldn't monopolise their cousin's time at her own party so it was a room full of noisy, excited children they didn't know from Adam.
That wasn't an ideal scenario for an autistic child with sensory issues like my child.

Autism isn't a reason not to have friends. It's very insulting to say it is. My (numerous) autistic family members all have friends.

A couple of other people have said this already, but sometimes the relationships we have with friends whom we choose are more nurturing, trustworthy and loving than those we have with our families.

I know it’s hard, being autistic, but I’d focus on that for your child. Choosing people she likes, and has stuff in common with and you facilitating and nurturing that through play dates.

Everyone is allowed to have boundaries. The SIL doesn’t want a relationship with OP and her son beyond a present exchange at OP’s house once a year and OP doesn’t want that kind of relationship.

People are bending over backwards to justify the SIL’s autonomy and yet OP isn’t allowed to have the same.

OP, the cousins will never have a meaningful relationships.

Reclaim your Boxing Day with your son, tell DH he can take PIL to SIL’s on his own.

And do not host the cousin’s birthday tea at your house ever again.

But cant you see that your autism is possibly making you over react to this situation and that your views on family and expectations are not "normal."
If you are aware and accepting that you have autism you need to be aware of how that can impact your relationships, you seem very rigid and only accepting of how you feel and not others but if you wsnt friends and want your daughter to grow up with healthy friendships and relationships you do need to reflect on your own behaviour.

If you are an hour away how would the cousins ever have been the best friends that you want/expect(?) them to be?

If you find it hard to make & keep friends why did you expect that SIL would be able to form a friendship with you?

Just because your SIL doesn't have the relationship that you think she should with her family, that doesn't make her a bitch.

No it isnt. If it’s a party full of school friends it could put birthday child in a position where they have to look after yours due to them not knowing anyone else preventing them from playing with all their other friends at their own party. That would be unfair to birthday child.

But even if your SIL and BIL were lovely and keen to facilitate a relationship between your two children, due to the distance your DD wouldn't be able to play with her cousin every day. Unless your child has a friend who lives next door, even close friends wouldn't be able to play together every day.

Can you sign your DD up for clubs and activities such as dancing classes, swimming lessons, Brownies etc?

I do think that your expectations are unrealistic. Is your child generally happy at school? Has the teacher raised any friendship issues with you? You may be projecting your own feelings of rejection onto your daughter.

Most parents worries about their child's friendships, especially if their child isn't very outgoing or confident. Most children do manage to form their own friendships with classmates. Your daughter did have three girls who came for playdates. That is something positive.

The party issue aside which I don't think they were unreasonable about, it sounds like a lot is off with the extended family relationships.

I agree with PP, the Boxing Day thing is awful. Why can't your DP and his parents save up throughout the year for a taxi journey so you can all go? Sometimes I think you do teach people how to treat you and by being ok with this exclusion are you suggesting to your DHs family that you aren't important to your DH so why should they treat you any better?

It also sounds like your SIL might have her own issues with her DH trying to limit her social life. It's hard to say and I doubt you and your DH are ever going to get the full picture of someone else's marriage.

All that said a lot of people don't have the extended family relationships that they wish they had. It's something that has to be grieved for if necessary and the close relationships sought elsewhere.

Completely agree with this. Take the pressure off.
My parents did zero social arrangement for me and I somehow managed to make my own friends, and sort my own social life when I was older

I agree that you just can’t demand friendships be run on your own time. I’ve disengaged myself from a lot of friendships as they have wanted a level of attachment and closeness that I’m not willing to engage in. I just don’t need it or enjoy it.

Isn't it pretty common to start splitting parties around that age. We certainly have for the last few years now - one school /football/other club friends party, one family party and some years an extra friends with children party. My kids get alot of parties!
Everyone we know does the same thing.

But the secrecy around the first party is the issue.

YABU for the reactions though. Its very OTT. Just step away from the relationship for a while. You can't force cousins to be friends.

I’m going to agree with this: if there’s one thing which is guaranteed to make friendships harder its the expectation that the relationship has to run exactly to your rules and requirements and that any divergence from this is an insult.

You seem to have decided that because they are blood relatives there is some commitment which they are held to on pain of discommunication

I know you have difficulty understanding this but most people, neurodiverse or otherwise, don’t see the world like this. People take different approaches to family relationships but most people would not expect a cousin to be automatically prioritised over all other friendships all the time, particularly when a child is getting older and making his or her choices about their social life.

I’m afraid if you pass this sort of rigidity onto your child you are setting her up for a lifetime of disappointment similar to what you have experienced.

I mean this kindly but please please try to break this sense of rigidity and entitlement. It is your daughter who is going to suffer.

Asking for your DN to go on holiday with you, making repeated offers, is OTT, OP. Your SiL doesn't want you taking her DC on holiday without her. Your DC and her DC are cousins. They will be happy to see each other occasionally. When they grow up they will make their own family. And you sound hard work.

Excellent post.

It is disappointing that your DD has been hurt by your SIL not inviting her to her cousin's party. It was definitely an unkind thing to do.

However, you do need to try and encourage and facilitate some friendships with children who are more local, such as school friends. Your daughter will need to be able to form friendships as she goes through primary and senior school, then possibly at University and in employment. Just relying on her cousin isn't going to help her build these social skills.

You said your mil is 'larger' so can she not sit in the front seat of the car and your pil in the back with you and DD? Even if he has a walking stick , it's the same getting into the back of the car, as it is in the front.

Sorry but you are the one who is going to ruin your daughter’s relationship with her niece. Once your children hit primary age life becomes busy, they start attending after school clubs and activities, if they do sports it can take up whole weekends.
I think it’s nice they took the time to have cake with family. I guarantee you they will not know they have done anything wrong.
The right thing to do was to pretend to be happy for her that she had a nice party. Be humbled that they took the time to have cake with you. Explain to your daughter that it was a school party and don’t give it another thought.
Every single poster is trying to make you see sense, and sadly you are not.

All of this. My kids have loads of friends but mainly only see them in school because outside of school is often overwhelming or they’re all sensoried out after the school week.
i have a low tolerance for socialising even though I have lots of friends I don’t see them all the time and I don’t have close relationships with any of my family, I think a big part of that is because they can’t relate to me as a neurodivergent person.
i always had things to do with my kids at the weekend, either me and the kids (single mum) me with friends and their kids or joining the various community events and activities over the years.
there are so many nd parents who also find making friends hard. It will make such a difference if you can turn your mind to something outside of this intense family anger.
theyre not even your blood relatives and it sounds like your dh might be asd too, so you both need make the effort to make a social life for yourselves or your just modelling sadness and isolation for your child

From the outside, it sounds like there’s been a lot of pressure on them - they’re your only option for family and you have high expectations of what a positive family dynamic should be like (unconditional love, very close with frequent contact and visits, etc).

All relationships are built on compromise and understanding. Withdrawing completely because they won’t do what you want isn’t compromise and judging them for how much contact they have with your PILs/calling your sister in law a bitch isn’t understanding. They don’t owe you a relationship, whether they don’t have the desire or capacity (your BIL advocating for his parents will occupy a fair bit of their time).

I come from a huge complicated family but my DH is from a similar v small distant family. We talk about “breaking the cycle”, with each other and our families, which to us means trying different solutions hoping for more positive outcomes. He’s rebuilt his relationship with his sister on this basis since we met.

If your desired positive outcome is more company/less loneliness for your child then you need to try multiple solutions - clubs/activities, speak to their school about any support/suggestions they may have, look for groups for ND parents/adults/children. Take control of finding the solution instead of pinning it on others with no agreement from them about the role they’re being given.