Need advice on bad behaviour

So I have a son who is 2 in July. He is very kind and sharing caring person. Of course nobody’s child is perfect but he actually shares toys with kids and with other kids he plays without crying pushing snatching you name it. His cousin is 3 and a bit and she’s done that since my son was born, pushed him, snatched toys, dragged him on the ground by his clothes, bitten him, takes his water bottle off and used to take his funny off, comes to our house and shouts mine on all his toys. He is never allowed to play with anything or if he finds something he likes he gets pushed and toys snatched off him.

My issue is, because her parents don’t address this much, I see it day in and out and I am fed up with it. My son is intimidated by her and it is ridiculous. He can play nicely with others and nobody else takes his toys or even shoes off and says they are hers, even thought they aren’t, and my son doesn’t play with half the toys because he says they are hers but actually are not. He is so scared of her and I personally don’t think it is a positive environment for not even a 2 year old. I can see she is a bully and attention seeker. She has a younger brother who is 6 months old, but she was a brat even before to my son. She hits her mother and doesn’t listen to many people, she screams if she can’t get her way and is just the nightmare child.

I understand all children are different and go through phases in life differently but this isn’t a phase. It’s been going on for as long as my son has been out of my womb.

WHAT SHOULD I DO? How should I address these issues with her? I am so close to losing my biscuits. We are going camping over the Easter holidays with them and i would like to educate myself somehow on your recommendations what to do please!! I don’t want my child to be bullied anymore.

He really doesn’t deserve it. Of course he has his moments too where he draws on the walls with a pen or finds my tampon stash and gets it all out, but he wouldn’t hurt a fly.. and I know I am not making this up, everyone sees it but nobody wants to get involved. It’s my partners brothers daughter. My partner has told his brother to do something about it and his brother thought I was just whinging and what not. Was joking about it behind my back to my partners parents. They see it , but they don’t want to get involved. My partner understands it and tries to say something, but he has been shut down by his brother a couple of times now.