baby bully?

My niece is not even two yet, but she is the meanest kid around.

My three and a half year old is flat out terrified of her. She hits and bites and throws toys at him constantly! If he has a toy, she takes it. If she has a toy, she breaks it. And yet, she is not my child, so I feel I cannot discipline her more than a loud NO!

Her parents try, but she ignores them the same way she ignores me!

I need some suggestions. It is not fair to Stuperman that he is afraid in his own home. I am tried of seeing him hide behind the hamper or run to his room and hide under his bed - just so she cannot find him and hit him again.

Sigh.