Nephew hitting daughter!

My nephew keeps hitting my daughter(2) they are four months apart. He is the third child, my daughter is an only child. I have no way to resolve this issue with my sil because she gets frustrated and smacks him over it and defends hitting.

I’m the complete opposite and feel like I am at a loss. I don’t want my daughter to be hit because the boy can’t have 100% attention on him all the time and I also don’t want her hitting others. I’ve been working really hard on peaceful and attachment parenting, I don’t want her to be influenced. What would you do? Limit visits? She loves her cousins, but I’m sick of her getting smacked and feel awful because then my nephew gets punished the same way.