My dog bit my niece

My niece (2 years) sees my sister be awful to the dogs like screaming at them and smacking them. My niece is like that now. She is constantly punching the dogs and screaming at them. I always correct her and put the dogs away when I’m there and my sister gets mad and says “she’s just a toddler”. Well today she dropped my niece off at my house and I told her to pick her up because I can’t keep her away from the dog but she kept running errands and making excuses instead of coming to get her. There isn’t a lot of space for me to separate her from my dog at my house since my house is only about 450 sq ft. Well she ran up and punched my dog and then pulled his tail and tried jumping on him and he nipped her hand and caused a bruise. He’s about 15 pounds. Now my sister is mad at me that my dog nipped her hand. She was told over and over again to not bother the dogs and I’ve been the only one constantly correcting her but it’s not helpful that my sister lets her be that way with their dog. My dog is in trouble but they want me to put him down and I’m not. Now they’re saying I’m not allowed to see my niece anymore