This might not apply to your family, but has been a solution in our home.
My husband and I used to have an appallingly rough marriage. We didn't speak to each other kindly or model healthy relationship.
Now that we are behaving differently towards each other, acting with love and consideration, modeling healthy conflict, showing affection, sharing time, space, etc., our children are acting very differently towards each other. Sure, they bicker, but they act respectfully towards each other much more often than not.
I try to really reinforce this through praise (ex: Wow, way to share kids! / I love the way you are playing with each other today! / Could you help your sister out with this please? Thanks, that was a really kind thing to do!)
I also give 'em help mediating their conflict and talking about their feelings. (ex: I feel ________ when you _______ (hint: I don't feel like you're a brat, I feel frustrated. I don't feel LIKE (that's a thought, unless we're in simile land) and you don't get to MAKE me feel, I have my feelings and I make them all by myself.)
It also helps in my home if I jump in sometimes, "Oh, Aria, you need to use your respectful tone and words. Talk it out, please find a solution. What can you do to help yourself feel better without being rude to Opal?"
If one hurts the other (hasn't happened in a while, huge step in our home!!) I'll say, "you're not being safe to ____ so you need to take your body away from her and have space."
If they speak to each other with raised tones I'll say, "Until you're ready to treat each other respectfully, you're not to be in the same room." They share a room, most of their friends, most of their toys, most of their clothes - and that's a lot to share. So they also have things they don't have to share unless they want to. Each has one sport they go to just by themselves. Each has a cubby in their room that they have total control over. Each has a treasure stash and a few toys that are 'extra special'. Each afternoon we have quiet play time where they have some room from each other and can listen to music and just relax. They WANT to play with each other, but they do better if they have some alone time each day. This helps them not feel suffocated by the other.
My eldest is also the most aggressive of the two, so I keep on her throughout the day. "Hey Aria, you don't get to speak in that tone to Opal. How do you feel when someone speaks to you/takes your toy/pushes past you? Do you want Opal to feel that way? No, of course not, you love her and want her to feel safe and happy. What do you need to do (apologize)? If you continue to act rudely, I will need to have you take space until you're ready to change your attitude.
I feel really proud of how far those two have come. In the mornings I listen to them playing with each other, helping each other out, trading toys and games. It's cool, because when my eldest (tech. my bio niece) came to live with us she was brutal towards my youngest. They fought a LOT and sometimes it was really bad. I'm so happy that they pal around with such respect these days. Good luck!