If you had a neice or nephew who always fought with your kid, would you keep them apart?

While it is common 2yr old behavior the mothers behavior is less than desired. I would say something politely to the mom before breaking contact or I would just stop the girl myself by telling her not to hit or scream even when it is directed at her mom. Mom may not Know how to handle the situation so will catch on and appreciate the intervention. Even if she doesn't it will show your child the proper way to act.