Questions: Family & Friends

I have been with my husband for 7 years and we have a 1 year old daughter together. I also have a 9yo son from my previous relationship and my husband has adopted him. His family has always been toxic but it's always been in a "I feed off drama" way and nothing more. However, they really upped it last year. Basically my sister in laws husband hit my 9yo son in the back of the head out of anger (my son was playing tag with his cousin and ran inside and the door slammed behind him). It was a hard hit. Cops were called and the police did nothing because there was no witnesses outside of me and they called it a "he said she said" thing and stated that we needed to work through our own family conflict. They claimed my son wasn't a reliable witness because he wouldn't speak to them (he's incredibly shy, has ASD and was just assaulted by a grown ass man so he was already scared - he just kept hiding behind me saying "mum can you just tell them" and they took this as me making shit up and leading my son to say something that wasn't true).

When my sister in law found out what happened she was initially on our side and scolded her husband. But he threatened to divorce her, so she did a 180° and started claiming her husband did nothing wrong and that he only hit my son because my kid has been repeatedly told not to slam doors and didn't listen so her husband had an "impulse reaction" and claimed "no harm, no foul" because my son was fine. So, I cut all contact and when our daughter was born we refused to let her anywhere near her despite her being extremely excited for the baby and having made all these customized items for her. She was destroyed over it but oh well. Don't defend a child abuser.

Well, here it is a year later and my mother in law brought it up randomly and stated "I understand completely why he hit your son." So I told her to leave and she wasn't welcome back here around my children. Now my husband is upset because he wants his mother around.