How do you deal with siblings and weight?

SpockAdventures

I am the oldest of four sisters. I've always been the chubbier one, and I'm sure that part of that is because I'm 6 years older than the next oldest, and 12 years older than the youngest. However, in the last year, my weight hasn't changed much, but all three of them have gained - and I'm not even joking - a minimum of 30lbs a piece. My youngest sister started college last year, and easily put on 60lbs. Even my oldest of the three, who has danced her whole life, put on like 30lbs over the last year.

I feel like a terrible person for it, but looking at them is part of why I am so dedicated to losing weight now. I think that the weight they have gained is...well....kind of gross (and I am so sorry if that is offensive, I don't LIKE thinking that about anyone, let alone my own family!), and I want to help them take care of themselves, but the oldest has undiagnosed mental health issues, probably bipolar like myself, so is incredibly emotionally unstable and can't even talk about MY weight loss efforts, let alone her own. The middle lives with her boyfriend and his family who are all over weight, happily so, and she just lives in that culture of fat acceptance, and my youngest....she's the one I'm most worried about...she is in plain old full out denial that she is overweight at all, when she would easily fall into morbid obesity with her BMI.

I want to serve as a positive role model, but just casually talking about my own efforts just doesn't seem to mean anything to them, or they just shut down and don't listen. I'm really good at tough love - like, I'm the one they come to when they know they need a reality check - but this kind of tough love could be really really psychologically damaging to generally unstable people already. (We have a run of genetic mental illness in our family on both sides....awesome for us, I know!)

TL;DR: How do you guys deal with siblings or loved ones that need help, either to gain weight or to lose weight, if you help at all?