What should I do?

So my nephew will be 3 in 2 months. My son is 10 months. They are 24 months apart.

My nephew is the youngest of my sisters two children. He has an older sister. Well ever since my nephew was born they have referred to him as “the baby” and treated him like a baby so that his older sister wouldn’t be rough with him. But now it has gone too far. By this I mean that years of being called “the baby” and being overprotective of him has turned HIM into a bully. He’s used to getting his way and not being told anything. Instead of his older sister pushing him around he does it to her. Pulls her hair when she has a toy that he wants, pushes her, hits her, bites her, yells in her face whenever things dont go his way. And no one ever tells him anything. This wouldnt be my business except that he now thinks it’s okay to treat my baby like this. Which of course he does, he’s never reprimanded at home for it so he thinks it’s perfectly okay to act that way. He’ll even yell at his parents and hit them and they just shrug it off, or if they put him on time out he’ll just say no and walk away.

So the last few times they’ve been over (they live in the front house of our duplex) or we’ve gone over there briefly he will act this way to my son if my son has a toy he wants. Or if we go over and he sees my son crawling toward a toy he’ll get in front of him, stomp on the floor, and yell at him. And if he actually makes it to the toy he’ll snatch it from his hand or hit him. Usually I make it before he hurts him but there’s been a couple times where he actually hits him in the face or pushes him over. I tell my nephew that he’s just a baby and not to hit him but he’ll just hit me instead. Im a firm believer in not punishing other people’s kids so I’ll tell his parents but they’ll just roll their eyes.

I dont go over much anymore even though we’re literally on the same yard and i do my best to keep my niece and nephew out of our house to prevent it. But still they find their way inside. If i forget to lock my door while we’re home they’ll just let themselves in. I honestly dont know what to do. I get kids are kids but he’s old enough to be reprimanded some way and understand not to hurt babies. Any suggestions? Am I overreacting? Any advice welcome