AIBU to be afraid to go near my abusive BIL?

[Worstsisterintheworldallegedly]

My sister's partner recently hit her during an argument and ran off with their baby while threatening to kill himself. She came to me for support and I tried my best to help her, looking at her immediate safety and not pressuring her about whether she would leave him.

After about a week she stopped answering my calls and I heard through my parents that they were back together. I was very concerned but couldn't get in contact with her so gave her some space.

Today I received an invitation to the baby's first birthday party, a small group of only close family including the abusive partner. I told my sister that I didn't feel comfortable going but would like to celebrate with her and the baby separately. She became extremely aggressive towards me and told me I was selfish and she is cutting me out of her life forever. Even our parents think I am overreacting.

Honestly the thought of me and my children being in the same room as him terrifies me. While I don't think he would immediately become violent in public I am worried he would be aware of my hostility towards him and start a fight that could become violent quickly. He is a big man, a former professional rugby player.

AIBU to be so afraid or am I overreacting? What should I do to keep my family safe but still support my sister?

YANBU at all and your family thinking you're overreacting after he hit your sister is alarming.

YANBU It's scary that your parents think you are overreacting even after he's been violent towards her! Do they know he hit her?

You are right AnUtterIdiot I do feel sick at the thought of seeing him with her. I keep visualising him hitting her in my head and my heart breaks for her. The reason I didn't say this to her was that I thought it was too emotive and would anger her. Sorry if it came across like this wasn't a priority.

My parents do know he hit her but seem to be minimising it, in my opinion. They said they haven't forgiven him but are willing to accept him back 'on the condition that he never does it again'.

I'd go purely to support your sister.

Until your sister is ready to leave all you can do is be there for her

Perhaps your parents are afraid of him too, hence the minimising.

She seems determined to stay with him so there is nothing you can do. If you are likely to show how upset you are then it would be better to avoid any social gathering for a while, so send a present for the baby but decline the party invitation.

Try not to bring the subject up with her, she clearly doesn't want to discuss it and is probably hoping it will never happen again.

I don't blame you for being scared but in my experience sometimes you can support people in abusive relationships better if you 'minimise' on the surface, if your at risk if being cut out otherwise.
Could you go but not take your kids?