To have nothing further to do with them?

My kids have a lovely relationship with their cousins. But we don’t invite them to parties. Why? Because they wouldn’t know anyone and then my child would feel like they had to babysit their cousin and not play with their friends and have fun. Also your child is a boy? And theirs is a girl? By 7 they are having single sex parties. They probably haven’t told you because of your crazy OTT reaction.
Im sorry you have an only child. I take it this wasn’t what you wanted? But being an only child isn’t miserable. And just because they have a cousin the same age it doesn’t mean that should be their auto support system.
Rather than flying off the handle. Perhaps you should have an adult conversation with your inlaws, I suspect they are keeping you very much at arms length because of these reactions

I think most people can understand OP’s underlying intention. But for whatever reason, her in-laws don’t see it the same way, and don’t want to push this relationship. We don’t know why, maybe it’s come from the child, maybe they’re always really busy, maybe they don’t like her. But whatever it is, they haven’t really done anything wrong. They just don’t want to meet up. I can understand OP being sad about this. But going “nuclear” is an overreaction.

After reading your posts, I think its probably you they are distancing themselves from, which obviously affects your DC. You sound intense and hardwork.

*edited for typos

Why haven't you asked them if there's an issue, instead of flying off the handle?

You are being very very over sensitive. My siblings fell out over something very similar, years later they both admit how ridiculous it was. Once the cousins are older they can have their own relationship, you can’t force one

Most only children build their own family and a network of friends. They do not forcefully attach themselves to other family members.

There are plenty of happy only children, where cousins feature only as part of a broader friendship circle. The closeness of the relationship is more likely to be determined by how much the cousins genuinely like each other than one of their mothers throwing tantrums.

You are your daughter’s worst enemy. Unless you start behaving like a rational human being, and allow a friendship between them develop naturally (or not, as the case may be) I can guarantee that they will want nothing more to do with you.

It is you who is sabotaging your daughter’s chances of a normal relationship with her cousin. It is you who is modelling terrible social and conflict resolution skills for your daughter.

To reassure you, OP, I have one female cousin. We saw each other roughly once every two years (they lived hundreds of miles away, and this was the 60's so no SM to keep in touch. My family didn't even have a phone!)

We were still very friendly whenever we met up at family occasions. When her last parent died (she's an only child), she increased the contact from her side and now we meet up frequently, holiday together and get on fabulously.

Contact or lack of as a child doesn't always mean no contact when you're an adult. And if they both marry and have children, they will make their own families and not be totally alone in the world, so you don't have to worry there.

If my SIL started talking about how they’d be each others only family and insisting they were invited to birthday parties once school had started I’d distance myself too

You can’t criticise them for not sharing pictures when I can guarantee that if they had shared pictures, you’d have accused them of rubbing your noses in it or something.
Whether it was them or their child who didn’t want to invite your child, keeping it quiet is the decent thing to do.

You might not have seen my replies but I believe I’m the only person on this thread who’s in a similar position to you.

Whats your DH’s take on all this?

I can understand why you’re feeling like this and I’ve said yanu because I think you should move on. But I also think you will regret being so black and white about this, as relationships just aren’t. I hope you get the opportunity to rekindle a relationship that works well for both parties. Perhaps you can admit you accidentally removed yourself from the WhatsApp group and be re admitted.

You tried to take their kid for an entire summer. You're badgering them.

You're not alone. You have a husband and a child. You may have been lonely as a child but a sibling is not your automatic cure for loneliness. You're trying to force relationships with other people's child and you're wildly overstepping.

Ahhh its sad but dc get to an age where they just want to be with their peer group from school & for that reason I can understand your dc not being invited to the party.
The rest of it isn't very nice but I think I'd reassure my dc & encourage them to form other friendships. You can't force relationships & they might well get on when they're older anyway!
Distance yourself quietly, going nuclear never helps anyone.

I think it depends what kind of party it was ..was it a big party in a hall with room for lots of people, or just a smaller group where there was a charge per person like soft play or something?

not very nice of them OP, I can see why you’re hurt.

I get the sentiment OP but it sounds like you have issues with rejection or fear of abandonment or something along those lines that I think you should explore.

This is an insane reaction. We don’t invite any cousins to our DC birthday. They were celebrating with you today

That’s a you problem. You shouldn’t make it a dc problem. And you’ve just gone nuclear and there won’t be any coming back from this so from now on you’ll have to leave it to your DH.

You have lost any control you ever had over it. And they’ll be even less likely to want to be around you now.

I understand the difficulties in being/having an only child. I understand you facilitating this friendship with their cousin. I also understand that many on MN are absolutely clueless about this.

But things do change.

Has your DH spoken to his sister about this - why things seem to have changed? Leave aside the party, which is normal.

I agree with you Op. It’s weird behaviour. Ask your DH to intervene.

What reason have they give for all of this?

Have you actually asked them?

I seems such a shame when the children are so close in age and cousins. Definitely a relationship I would encourage.

Just because you think they should be stuck together like glue and be automatic friends and playmates because cousins doesn't mean your SIL agreed to that. They're distancing themselves from you because you're being unreasonable and I don't expect that will change since you have gone defcon1 over their choices for their child.