When one of your kids is using rough hands or truly hurtful words with their siblings, they need you to set the boundary.
And they need you to set it while offering compassion towards both parties.
The things we want our kids to learn about conflict are:
1. Safety and boundaries (fair fighting)
2. Effective communication of their needs
3. How to move to repair
When kids are in conflict and no one is being hurt physically or being subjected to truly demeaning and cruel verbal abuses, it's absolutely ok to let them work it out. In fact they need that practice.
But when things are below the belt, our kids need us to set the limit and enforce fair fighting rules. As their parents they need to know that we care when they are being mistreated and we care when they are so upset they have started mistreating someone else.
Keep as calm as you can, give empathy to everyone, and know that their conflicts are important learning spaces.