Disciplining other people's kids

This has been bothering me for a while. What are people's thoughts on disciplining other people's kids when the mum is there?

I was at my partners mums and his sister and nephew were there. My 4 month old was playing with her toys and his nephew kept taking them off her. His mum kept saying to give them back but he ignored her and carried on.

Would it have been OK to step in as it was affecting my daughter.

The same nephew (same day) also pulled my daughters hair because she managed to get hold of his. I wasn't holding my daughter at the time my partners other sister was. She told him not to do it but it wasn't exactly telling him off just said she is a baby and doesn't understand (it broke my heart watching him do that to my little girl)

The nephew is 4 years old and doesn't listen to anyone. No one seems to discipline him and he is a very naughty little boy. My niece is the same age and would never behave as he does