Undermining my sils parenting

My niece is 15 and when she is with us we honestly ignore my sister in laws food rules and only listen to what my brother says because my brother isn't basically trying to kill her. My brother and his wife have always disagreed on my nieces diet. My SIL insists she fat whereas my brother says she's not. She's 5'5 and 107 pounds btw. She's a skinny minny. My brother and his wife would constantly fight over this. She has told my niece not to eat something and my brother will tell her to stfu and she's not fat. They're on the brick if ba divorce at this point. My SIL rules are no more than 700 calories a day. My brother says that's stupid and let her eat what she wants. She came to see us yesterday and we were trying to get her to eat and she started crying asking if we thought she was fat and we said no. She did eat and seemed happy and here comes sil asking if we kept track of how many calories she ate. We said no and she eventually realized we didn't count and said that she's her daughter and we aren't the parents. I said my brother is an equal parent and we listened to what he says because she's trying to make her anorexic if she isn't already. So my SIL is saying we can't be at around her then. My brother said he's gonna handle it. His wife also eats whatever she wants and definitely isn't as thin as my niece but she pushes this on my niece.