I don’t know if I should feel mad

I don’t really know how I should feel about this. A little back story my nephew is 5 and my baby is 10 months old. He loves but he does little things to hurt her. Not to make marks or even make her cry but things he knows he shouldn’t be doing. His mom gets him in trouble but he continues to do it. Today my baby was crawling on the floor and when she sat down he pushed her and she feel face to the tile floor. She got a bump that turned red. I do not live with them but I visit often. I let my emotions run and I told stuff to his mom. Like what he did is not okay yea you got him in trouble but I am not ganna wait til he does something else to hurt her. He could of cracked her head open. I don’t feel like going around them anymore. She’s my sister and we are very close but I just feel mad or emotional. I see my baby’s red bump and I just feel bad. Would you feel like same?