Am I the worst MIL?

With your update OP, I think the best thing is to do whatever you can do with the best grace.
If you decide not to go, then consider wording it like this "It's so kind of you to invite me along, but I hope you don't mind if I don't come as I am not the best with big groups, so let me take DS and you out for lunch next time I come through."

Be careful though, if it's a big family group who lives close and just you on your DSs side, this may be how it's going to be going forward. For everyone's sake, but mostly your own, it might be worth going along with an open mind and trying to get to know people.

In your position, I'd go. Suck up the conversation from the dotty aunt, smile and try to build on your relationship with your son and DIL.
What's the worst that can happen? You'll have wasted 6 hours of your life trying to be involved in your son's birthday celebrations.
If money is an issue, can you talk to your son and DIL? Or borrow some? I'd rather spend my last £50 on seeing my child than anything else.
I'm sure they'd rather see you than not.
Surely better than be at home ruminating about what's going on....although you say you have plenty in your life. But this is important, isn't it?
Sometimes you have to do things for the greater good.
However, it's up to you.

I disagree. Love our SIL but his family aren’t ours.

@MrsSkylerWhite yes they are family, through marriage. I would class them as extended family. They are all immediate family to the OP’s son and the dinner is for him and his family.

The other guests aren’t OP’s family.

I think it would be good for you to go. You will probably have a lovely time and it is good for families to spend time together. Do you think social anxiety could be driving this? If so, I would do my best to overcome those feelings because declining invitations can become a habit, and then socialising becomes much harder.

I wouldn’t go either. Sounds awful.

I’m a bit confused why you’re approaching this as a MIL issue, when it’s a mum issue.

Perhaps a little irrelevant but just wondering why you can't see your son alone?
As in, what would happen if you text your son saying "do you fancy having dinner next Saturday, just us 2?" And if any questions asked you can always say you'd like 1-on-1 time with your child.

I only have DDs, but 2 of them have other halves and i'm trying to put myself in your position.

If one of my DD's husbands had organised a meal for DD's birthday - and invited me and a big group of his family - would i go?

hmm

First i'm wondering - how come there isn't more on your side OP? How come just you and a big group of them? Are you an only child? Is your son an only? Has he no grandparents?

Secondly i'm thinking - how will a 'big birthday meal for the whole family' look if his mum doesn't turn up ? Bit sad.

I think i'd go.

I don't particularly like the parents of one of my DDs husbands, but I'd drive an hour and a half to show my face at my own DDs birthday meal.

Lastly - is it weird that he doesn't want to meet you alone? I don't know. My DH wouldn't want to meet his mum alone for a meal either. He loves her but he'd like us to go as a couple as a) travelling companion and b) his mum can talk the hind legs off a donkey and he goes off into a dream😂

The travel alone would be enough reason to say no if you feel you need a reason. I do think though if you can see SIL one to one you should be able to see your son one to one too. Personally I would have been quite happy for my then DH to sometimes see his parents just on his own. If I knew you lived that far away I certainly wouldn't be questioning you as a mother for not coming to dinner. No reason you can't celebrate at a different time, its a birthday not a wedding or you meeting their baby for the first time.

The ironic part is your poor dil may well think that you are not her cup of tea either but has to tolerate you for the sake of her dh... think about it.

I think the drive alone makes you definitely not unreasonable! Just say you will happily organise something with just yourselves at a date and place closer to you, I doubt she will mind

I think it's a bit poor to not go to your son's birthday celebration to be honest. If the in laws were awful and rude and mean then 100% don't go but if they are just not your type then I think once a year some effort is required. I do have a mother that makes very little effort though and I do all the running which will be clouding my judgment of course!

It's interesting that you say am I the worst MIL - this is about you as a mother not going to your sons birthday celebration not you as a MIL as it's not your DIL's birthday I would suggest so it's an odd title. If i was one of the party who did go and was judging you for not turning up I would judge you as a mother not MIL.

Don’t go if you don’t want to, but then don’t come on here crying about how the (future) grandkids are so much closer to the other set of grandparents

Perhaps the 'pleasant enough lady' isn't actually bothered and has invited you because she loves your son and wants to make him happy?

That was a prickly defensive post op. Why do you think your son avoids doing 121 meet ups with you? It’s easy enough to arrange, do you suspect your dil prevents them?

A good way to ruin the perfectly civil relationship you have with them, is to start to withdraw and cease making an effort for special occasions. It’s your son’s birthday, most mothers would love to be invited to a celebratory dinner.

Instead it is coming across that you can’t be bothered with it. A sullen indifference even. This dinner is not about you though, it’s about your son. How do you think he will feel?

If you can’t be bothered to see him on his birthday, despite being invited, then I think speaks volumes, at least to me, what kind of mother you are.

Your son is likely to feel hurt, your dil is likely to feel somewhat rejected as it was her idea to gather both sides of the family.

We are of similar age I suspect, so I don’t buy the time of life line, you are making a rod for your own back with this one, and you can’t be surprised if they too decide to stop their own efforts and your invitations dry up altogether.

Your last post is coming over as bitter, resentful and difficult, I would suggest you explore your feelings more carefully or you may ruin your closest relationships entirely.

I suspect you had an adoring son, as a single parent, and now he has moved on with his life it has left you with some feelings of loss op. Really it would benefit you to really think about why you don’t want to go, without judgement, and look at why you feel like you do.

I would:

Say to be honest, if I'm making the trip
I want to get quality time with two of you, would it be ok if we went out another time (my treat)?

And separately, maybe in a couple of months:

DiL, you know I love spending time with you both, but for a one off (for Christmas?), would you be able to set up a meal with just me and DS? I know you shouldn't have to, but you know what he's like and it won't happen otherwise

You sound like you two have a good relationship.

I also agree with @Savingthehedgehogs.

It is your son's birthday. It is worth making the effort for.

It's your son's birthday and your DIL has arranged a nice meal that includes you, so I'd go to be honest. It's once a year!
It sounds as if your DIL does an awful lot for your son and you'd probably see even less of him if it wasn't for her arranging everything.

As so many others have said I believe @Savingthehedgehogs has it.

There is so much going on underneath this. However I am not sure of your willingness to think about that.
In short your DIL is your nexus to your son and it’s important you make an effort for a few hours. I doubt they are card-carrying fascists or violent criminals or some thing reasonable as a get-out. Not your cup of tea is NOT.

Completely agree with all of this.

I agree.

I wonder if your histrionic headline, obviously inviting the sought for response that, no, you’re not the worst MIL, might offer some insight into the dynamics here.

Is it really about your DIL?

Would a MN consensus that you’re not the worst MiL justify your feelings to yourself?