Last summer my brother's wife left him. He initially tried to hide the reason why but as it turns out he had been abusing her and she finally left him over his treatment of her oldest son. They have 4 children together (8M/5F/4F/0M) and she has an older son (16M). After finding out the reason for separation my sister and I no longer have a relationship with our brother.
My dad & brother couldn't care less to have found out that their son/brother was abusing his partner and have been painting his ex the one to blame. My mom is having a hard time. My brother has always been her favorite child and although she has become a lot colder with him she still has a relationship with him.
When SIL left my brother, it was the start of the kids summer break and she took them to stay with her dad in a different state. A month later she gave birth to her youngest child. When my mom found out the baby had been born, she was upset that SIL hadn't contacted her as she had been present for her other kids birth. When she tried to contact SIL she wouldn't answer her calls/messages.
In august, SIL moved back to our city so the kids could return to the same schools. Her and my brother have also sorted out custody/child support and my brother is only allowed a few hours of supervised visitation a week. Since then, my mom has been smothering SIL. It comes from a good place; she feels guilty about what her son did and wants to support SIL. She has been going to all the kids’ events/sports games, taking them out regularly, spoiling them with gifts (I think she spent close to $1000 on the 16 yr olds birthday), and has been inviting herself over to SILs house a lot.
I know SIL is grateful for the help I think she finds my mom's presence a bit much. She recently told my mom (while I was present) that she didn't want to see her until spring break and that afterwards they would sit down and create some boundaries in their relationship and her relationship with the kids. My mom was very upset and called my SIL ungrateful for everything she had done for her and that she would struggle as a single mom without her help. They argued for a while before my mom and I left. In the car my mom tried to get me on her side, but I told her I agreed with SIL and that she had been smothering her and the kids too much and they needed space.
A few days ago, my mom and I got into an argument about it again. My mom tried to defend herself by saying something along the lines of "it's not my fault \[brother\] abused her" to which I responded that she did hold some blame as she was the one who raised him and arguably helped program his sense of arrogance by favoring him. My mom is now angry at me that I would take SIL's side and overreacting about what I said. Even my sister thinks I'm in the wrong. AITA?