How to help children with low self esteem😔

I have a 7 year old sister and an 8 year old cousin who play a lot together. However he is so mean to her. He can bee really mean and manipulative which I hate. I feel like he’s made her self esteem go down or he makes her feel really bad about her self at times.

I’m not around a lot but when I am I make sure to watch her. But I feel like I should be doing more to prevent her from being hurt by him. I also think he could be hurting her in other ways other than emotional.

How can I talk to her and let her know not to let him get to her? She is a beautiful little girl and I would hate for her to grow up the way I did and hate her self. I want to teach her self love more than ever but idk how. I also want to ask if he’s hurt her in other ways but idk how to either. Please help