As I said earlier "I do wonder if they just don’t want my older, complex needs son around which would make me very, very angry. I can’t work out why they don’t care about my 4 year old?" - is a stupid comment. And a very entitled and pointless one.
Anyone can not want to be around your complex needs son, and you can get as angry as you like, but they will still make their own decisions based on their own children and their own needs, not your wants or enraged demands. In fact, the more you get "very very angry" about people making their own decisions the more you and your complex needs son will be avoided.
Not wanting to be around a 4 year old is completely normal depending on the age group of the kids.
And as mentioned, a few weeks ago there was a thread here by a very stupid woman who has destroyed her relationship with her adult daughter. Like the OP, she tried to force her daughter to be the unpaid support person for her son and like the OP would not allow her daughter to have a life, friends and interests without him.
After decades of this abusive behaviour that OP then shrieked viciously at her adult daughter because she would not leave a celebratory weekend with her adult friends and run to visit her brother, per her mother's demands.
The OP there wouldn't listen just as the OP here is refusing to see what she is doing.
This is a well recognised dynamic, where parents of disabled/high needs children try to cram them into their other children's lives.
Your children are not his parents. You are. And only you and your husband are responsible for him.
Life is tough and EVERYONE has their struggles. Your son is his parent's responsiblity and you cannot force others - not even family - to want him around.
If you keep this nonsense up, your daughter and your four year old will lose out. You will not be able to force anybody to include our son, unless they choose to, though you may manage to ruin lots of get togethers, outings and opportunities for your other children along the way.
And once they grow up and realise that their mother tried to force everyone to include their complex needs brother in everything rather than letting them have their own lives and friends, your other children will walk away from you.
The other OPs daughter has gone non contact with her.
Stop trying to control how other people feel, act and who they hang out with. Family included.
You have three children, not one, and no matter your own personal feelings, your complex needs son does not rank more highly than the other two in terms of rights. They are not an amorphous blob. They are not his carers. They are individuals.
And do not pretend your children are not well aware of your stance on them not being allowed to enjoy themselves if the rest of the family is not also invited.
It is abundantly clear that this was NOT a whole happy family event and only you and your husband two children left out. Not at all.
From now on when one gets an invitation just be happy for them and never assume it must include anyone else, and that goes for your complex needs son too. Sorted.
And if you want to see your other family members, invite them to stuff at your house and build a normal, organic relationship with them where you are not "very very angry" at someone making their own choices based on their own family's needs and wants.