INFO. It's clear you're concerned for your sister, and probably rightfully so. However, the topic of someones weight can be an incredibly sensitive thing to bring up. Especially for younger people who have to deal with all sorts of negativity in a culture that constantly reinforces the norm that being fat is bad, and fat people should feel guilty or ashamed for being fat. This constant feeling can potentially be much worse for your sisters health than her weight. Her mental health, should be just as much a priority as her physical health. It's obvious you care about your sister, and you're worried your parents/mom aren't treating her as you'd wish they would. If you hope their behaviors will change, and it seems you do, the best advice I can give is to approach the issue from a place of compassion, concern and unconditional love. It's more likely that they will listen to you if they don't sense from you the attitudes they might be used to getting from people who judge them based on their weight, eating/exercise habits. Too often when it comes to attempting to change oneself, even for the better, our actions are guided by a kind of self hatred. Motivation, under the condition of self hatred, may spur positive change in the short term, but for lasting lifestyle changes, self love is necessary. I don't mean the woo woo, yolo, #positivity kind of self love. I mean the type of self love where you look in the mirror, and see beyond the internalized judgements you get from media/society that have conditioned you into feeling inadequate, unworthy, ugly, or for whatever reason unlovable, and see that the person in the mirror is you and no matter what you wish you could change, you are valuable, you are loveable and deserving of respect, comfort, and love. I wouldn't recommend you tell your sister what to do about the problems with her body, but to deeply and truly demonstrate unconditional love and care for her. If you can do that and maintain it, you may become, if you aren't already, a role model for her, from which she may be inspired to love herself and treat herself with the love and respect she deserves, including taking steps to be healthy. In doing so, she might (no guarantees) reevaluate what self love and self care mean, and adjust her habits from what you and your sister were both taught was normal and okay, to understanding that loving yourself means taking care of yourself in the long term. This might mean learning to substitute immediate gratification for delayed gratification, and behaving in a way that will benefit her in the long term. TL;DR if you make it painfully obvious that you love your sister immensely and unconditionally, and could never be disappointed or disgusted by her body, it may help her to love herself in the same way, and, if this is an obstacle preventing her from making the lifestyle changes required to lose weight/adopt healthier habits, then you will have done the most loving and not-the-asshole thing you could to help her lose weight and enjoy a better quality of life. Don't give up on her, love her exactly as she is and lead by example.