Your situation sounds very hard. In your shoes I would rely on the aunt and uncle more and let your MIL and FIL know you will be doing this and why.
Please do not worry about the other parents at the party judging you, it's really unlikely to be an issue.
I hope you find some way to work things well with your inlaws. I do not think you are unreasonable to want to do things with just inlaws, and I think you do need to make it clear they are making this impossible. Yes, SIL is a problem and I would want to say the same to her, in a nice way, but your inlaws are allowing it, as you rightly say they are enablers.
Your SILs comments to you are awful and you have my full sympathy because she has said and done some very hurtful things.
Raggydolly although I do feel a lot of sympathy for your situation I find your postings about your SIL's adopted son really odd.
"He was a lovely little boy when SIL first got him"
Adopters do not usually say they 'got' a child (I'm an adopter), we usually say, when ds came to live with us.
"we could not believe she was approved" you sound like you really dislike her, I can understand why but surely that was over 6 years ago; I wonder if she can feel this dislike.
"She hit the jackpot, he was fostered from birth, no abuse or involved at all, he came to her at 18 months and settled straight in."
Again your way of talking about an adopted child joining the family is very odd. My son is brilliant but I've never said I 'hit the jackpot'; because his joining us involved him loosing birth family and a foster family. I am not saying you say this to your SIL but it just sounds quite odd to me. Like you think she got this perfect little toddler and somehow ruined him. Despite what he says, which may be his true feelings or not, adopted parents are not necessarily responsible for how their adopted children 'turn out.'
"Now he is just out of control, he spends most of his time with the inlaws."
Adopted children do sometimes go a bit 'off the rails'. This is not always because they are being parented badly, but sometimes because the damage done to them in the womb, in the first days/weeks/months of life are really very hard to combat with even very good parenting.
I hope you do not mind me mentioning this, it just stood out for me. 
Hope things work out better.