Not sure what to do

So I’m going to post this anonymously because I don’t know if my sister uses this app and I definitely don’t want her to see. But I watched my niece today. She’s about a year and a half old. I don’t think my sister is feeding her! Her doctor told my sister that if she didn’t start gaining weight (she’s literally still in 6-9 mo clothes) that he was going to have to call child protective services to find out what was going on. But literally every time I watch this child for the day she acts starved when it comes to food, she over eats as much as she can and still wants more and since I started taking her on a somewhat regular basis she’s kinda starting to grow.

I’m just wondering if maybe I should call child protective services because honestly this is bullshit. I’ve struggled financially before and still found ways to put food on the table for my kids even if it meant I didn’t get to eat that day, and financially they are doing better than me and my husband so I truly feel there is no excuse and it isn’t fair to my niece. I don’t want to call CPS on them but honestly what other options do I have? I’ve tried talking to my sister about this but she just lashed out and goes on the “you think I’m a bad mom” pity train so I can’t get through to her that way.