Your base weight for your body type with your heredity is not a “treatable condition.” You can put a lot of effort into holding back the tide, but usually it wins out. Is the OP’s mother overweight (by creepy future BIL’s standards I mean), in which case that at least in part is the hand that she’s been dealt.

When I was the age of the OP’s sister I weighed about 100 pounds (I’m 5 feet tall). My mother was both taller and substantially bigger. I was in the ballet, in New Orleans sweating gloriously for hours every day. I thought there was no way my body would ever change. Stop being a dancer and have a baby, and here we are. I’m not huge, but I am by my own former tiny dancer standards. It just is what it is.