This relates to my brother who is in his early sixties. He's been with his partner for three years and has had a very turbulent relationship throughout, with frequent rows over her not getting what she wants, and have had several occasions where they split up and then later got back together a month or so later.
She is a very toxic abusive person that none of the wider family can stand. Would you force yourself to be civil to her at family events for the sake of your brother or would you ignore her? His adult children particularly dislike her as she has stopped him spending time with them so they all ignore her.