What a difficult thing to decide when to get involved and when to stay out of it. I really believe it could take the wisdom of Solomon to know, as each situation is different, and we sometimes don't know the whole story.
I see two of my kids going at it hammers and tongs, and all the kids are rallying around one of them, and coming down hard on the one who overreacts, while saying relatively little about the one who provokes and throws his weight around. BOTH the kids involved are at fault in different ways, and I sort of wish that the other kids would stay out of it.
With me and my siblings (four of us total), a screaming rage aimed at me at a family gathering, followed by my dad (who has been to jail for domestic abuse and tried to choke at least two of his daughters) lecturing me on how to be a better parent, were the last straws for me. Two siblings tried to 'fix things' by lecturing me some more (can you guess what my family role is?) about how to be a better human being. I guess they each told my dad and the screaming sibling to look at their behaviors, but as long as they wouldn't, the default setting was to tell me again that somehow it was my fault and I should fix it.
As you can imagine, that type of fixing it didn't go over so well. None of them speak to me now--so I guess you could say they've withdrawn. Admittedly I make no effort to speak to them, either. I have withdrawn because I see no way to fix it so that it is peaceful for all of us, myself included.
So...do you fix it or stay out? I don't know. I feel for you and wish you the best of luck with sifting through the stories here to see what best fits your situation.