I need to get something off my chest... I can't say too much since I'm not a fly on the wall at my…

I need to get something off my chest... I can't say too much since I'm not a fly on the wall at my sisters house BUT I have noticed some things about my youngest niece that worry me. Not too long ago it was discovered that my niece was having some issues health wise and they aren't exactly sure what's going on. She was passing out in class and she's losing a lot of weight, she also has a very high heart rate. At first they thought there was something wrong with her heart but realized that wasn't an issue and started to think something was wrong with her major blood vessels in her neck. I haven't heard anything more about her but in all the photos I see on Facebook, she's just losing more and more weight, she's in cheerleading and her uniform barely fits her. She looks like a skeleton and she's only 12. I'm irritated because she is still going on with all her competitions and she's on the varsity team with high schoolers. She is pushing herself so much and it cannot be good for her, but she just keeps going. I don't really know if it's by her choice or my sister's. My sister has always pushed all her kids in their sports to extremes and I always thought it was ridiculous... I'm just worried about her and I know it's none of my business because it's my sister's life and her family... but she is my niece so I at least have a right to worry.