How to deal with immaturity and drama?!?

So I have this cousin who is I seriously think psycho and she doesn't like me cuz every boy she liked didn't like her and liked me instead. But she and her parents don't like me cuz she was trying to fight me and I fought back. But her parents and herself are so immature idk how to deal with it. They act like children worse if even. They will make faces and act dumb towards my son who has nothing to do with it. A couple years back they threatened me. Her mom said she'd attack me when least expecting it and her daughter threatened to kill me and my son. How the hell am I suppose to deal with these people??