Can I please ask what your / normal expectations are around siblings fighting?
I have two sons; 10&14, youngest has ASD&ADHD (which is relevant in the sense that i likely more protective given some of the figures around mental health). The boys fight, very often becoming physical, every day. My teenager is tall and athletic and probably doesn't recognise his own strength at times, although I do belive he does hold back from what he could do. Teen is difficult at home, glowing at school and his team sports. He is very hands on, he loves to squish my youngest cheeks (i know that sounds odd!) And arms. This can wind my youngest up as it can go on for too long or be painful. Teen says there is favouritism and can get wound up by me. I still treat them the same in almost all ways, including bed making and dinner making and lifts everywhere. But I think Its normal to have different expectations for them given their age etc. Such as expecting Teen to leave youngest alone or just sit and chat. Teen is also disruptive if youngest and I are watching a film, he'll come down, start messing around, squidging cheeks and being loud.
I don't know how to put this into words really. I just hoped that by now there would be no fighting. I hate confrontation of any kind, I sometimes freeze when they fight, I can't believe what's happening.