Hitting

So my daughter is 10 months old and I’ve been taking her to play with her two cousins more recently (the boy is almost 3, the girl just turned 1 so it’s perfect to get the girls together). The boy has some issues and seems like he has some deeply rooted anger. The kids were playing today and I watched how he hurts his sister, pushing her, squeezing her, digging his finger into her eye, taking her toys etc. So then he started doing it to my daughter. He pushed her and she landed on her face and started screaming. Later he kicked her in the stomach with his sneaker. And just kept taking every single toy away from her and trying to make her cry. I flipped out and grabbed her after each incident and sternly told him NO. What does his mom do? Sit back and chuckle and say “silly boy you can’t do that that’s not nice” yeah, that’s really going to teach him a lesson. I asked her why she doesn’t want to punish him and she said she doesn’t know what to do. So long story short, I don’t feel comfortable to be the one discipline someone else’s kid even if it is my nephew. But he needs to get disciplined. Since they’re family I’ll see them pretty often and I’m so afraid to let my daughter more than a few feet away from me Incase I need to scoop her up. I’m going to talk to her and tell her next time we come over if she can’t discipline him and if he hurts my daughter once we are leaving. But I don’t want it to get to that. Does anyone have any advice on what I should do or what you’d do? Thanks!