How do I stop my family from hurting my son?

[Marie]

Hi. I have had a rough time with my mom and my sister and her husband since my son was a few months old. My sister's son was the baby of the grandkids until my son was born. Well my sister would allow her son to punch my son in the stomach and hurt him all the time even when my son was a baby. I would grab my son and get onto my nephew and my sister wouldn't do anything. She would just make up an excuse that my son had hit him. Helllooo. He is a baby. Well, my mom would act the same exact way. My son is very sweet and is sweet to everyone and all the kids now except for my nephew whom he will hit at because my nephew is always hitting him. I had to keep my son away from my nephew for the longest time because he still tries to hurt him all the time. I'm upset at my mom and my sister. They still baby my nephew and are always getting onto my son(who doesn't understand why they do that). I'm tired of the situation and I don't want my son hurt.
I now only take my kid over during family functions and I keep watch on him at all times. So family functions are never fun. This past 4th of July was not fun at my families. My mom yelled at my son over top of everyone to stop hitting my nephew. I was totally keeping an eye on them and my nephew happened to walk up to my son and that was it. I had been keeping him away from my nephew. I told my mom that he did not hit him because I was watching them and it ended up a big scene. My mom and I already went through a big blowout but it evidently didn't mean anything. I quit going over there for a long time. The problem is still there. Do I keep him away completely?
Thanks so much for your answers, they really helped me. They really gave me the encouragement that I needed to see that they were acting psycho and immature and it is ok to stay away from your weirdo family. Thanks again.