If you feel "safe" meeting in-laws (nothing abusive etc on their side), then think about it as a celebration of your kid and that your presence will be highly appreciated.
I have already been struggling with how to behave when I will get the same kind of invitation when my D will celebraste her birthday (October). That I don't want to go and pretend to be a happy family etc. But that it would be something I will do for D, not W.
But do think about it and try not to make a desicion based on emotions. Just think it through a couple of times first and you will find the right answer for you.
H-30s W-30s M-5 T-10 D4 ILYBNILWY/BD-May/17 W moves out-May/17 D filed-May/17 House sold, move to apartment-Aug/17 D going through-Jan/18?