AITA for not making my son apologize to my niece after making her cry?

Original story

I’ll start off by saying every week, my sister hosts a dinner party at her house every Saturday. We are not from America, but our kids are born here, so they enjoy foods such as pizza and burgers.

I usually don’t want to come empty handed, so I always ask my sister if I could bring pizza for the kids. She always agrees and has no issue with it.

I have noticed her daughter avoids eating any of it, and whenever I ask her if she’s going to take some, she always responds with “I don’t eat processed junk.” Her mother is aware of her saying this but never corrects her behavior.

This is the reason I always ask her mother if it’s fine to bring, and yesterday I mentioned that her daughter doesn’t seem to like pizza. She says it’s fine to bring it anyway.

Yesterday the same thing occurred. I offered her once again, and she made a rude comment.

This time, however, my son responded to her. He tells my niece that she can go eat grass like cows if she’s so healthy.

Other kids were laughing, and some adults mentioned how the little kid is protecting his mother.

This results in my niece crying, and her mother took her to the side. The entire dinner night passed with my sister not saying anything, until after.

She calls me and says that I should have made my son apologize for his insult. At this point I’m also irritated, and I tell her the past few times her daughter has made rude and unwanted comments right in front of her, she has done nothing to correct it.

She then says that her daughter has never made my son cry and embarrassed her in front of everyone.

Both my parents agree that my sister needs to correct her daughters behavior.