Your niece is lucky to have a caring, emotionally intelligent aunt.
I agree with many of the comments already made [apart from Lesley Madden]. However well-intentioned her parents are and whether it's for health reasons or something else ... any shaming type comments around what their daughter eats could affect her self esteem and mental health, leading to her developing addictive and controlling behaviour around food.
Is it possible you could have a 1:1 chat with your brother first, telling him you are coming from a place of love for your niece and would like support with how to approach the topic with his wife/them both. Maybe even show him some of the responses above &/or find and direct him to helpful websites about how to sensitively address teenage weight issues e.g.
https://lindastade.com/talking-to-child ... ight-gain/ Depending on how that goes, risking confrontation with a repeat conversation with your sister in law may be helpful.
A kind responder above suggested asking your niece whether her mother's comments are bothering her. Instead I suggest you may gain more insight by having wider 1:1 gentle conversations about how she is, how is school going, what interests her, feelings and worries, etc.
Good luck!