My brother left his wife and young children a few months ago and moved in with OW. OW is pregnant.
My brother is telling my family all sorts of stories about how terrible his marriage was and my family have all bought right into in and have completely disowned my SIL. They are spreading the stories by brother is telling and trying to turn other people against her. I have supported my SIL throughout this and tried to stay neutral with my parents, but my mum has not taken this well. She’s made my life a nightmare about it and stopped speaking to me for a while.
But the bottom line is that I don’t feel the same way about my brother. I don’t want to sit and play happy families with him and this new woman and her DC and new baby when it arrives. I can’t excuse what he did. And the family is already divided over this because of the way my mum has tried to force me into taking sides. My brother has come up smelling of roses somehow 
I know it’s not my place to judge my brother, he has to live with the consequences of his decisions. And it’s nothing to do with me. But I can’t be the same with him. And I don’t want anything to do with the new baby. Does this make me a terrible person? Am I being unfair? It’s not the baby’s fault. It will be my niece/nephew and my baby’s cousin. I am very involved in his other DC lives, but this is different to me.
Full disclosure: I am 8 months pregnant so might not be thinking straight, and I’m having trouble getting my thoughts in order.