My DSis and I both became very overweight in our early twenties and we used to talk about it a lot, because we were both in the same boat, so it was easy to bring up and to discuss.
10 years ago we each decided to make a proper attempt to lose weight together. We supported each other, talked about diets and exercise constantly and both lost significant amounts until we were both in the 'healthy' BMI range. Since then, I have managed to maintain my weight, but my DSis has regained all the weight she lost - plus about another 3 or 4 stone. She's now around 25 stone, and very unhealthy.
Initially, when she started to put the weight back on, we would discuss it and I'd try and be supportive, but as time (and the weight) went on, it became very difficult to raise the subject with her. If I mentioned her weight or suggested a healthy meal I'd tried, she would get really defensive and cross with me - until it got to the stage where I didn't dare bring it up. Our DM got the same reaction, and we both became really worried about her health.
DSis met her DP a few years ago and they've just set a date for their wedding next year. DSis set up a Zoom call with DM and me last week and announced that she wanted to lose weight so that she felt her best on her wedding day. She was telling us because she wanted our support; DM is also keen to lose some weight so will be working on it with her and I will be cheering them both on - DM and I were so pleased for her that she had made the decision, because there was no way we could have broached the subject ourselves without getting our heads bitten off and my DSis feeling ganged up on or attacked.
Back when DSis and I were both very overweight, talking to her about weight loss was easy and was well received. Once I was a 'healthy' weight and DSis was morbidly obese, she did not want to hear anything from me about her weight. I am so relieved that her weight is now open for discussion again, as DM and I were becoming increasingly concerned about the affect it was having on her health - but as PPs have said - she is well aware that being so overweight is dangerous, so bringing it up with her would do nothing except cause problems in our relationship.
However, I am an alcoholic (I've been in recovery for nearly 8 years), and if DM and DSis (and DH, DF and others) hadn't taken the risk of it adversely affecting our relationship by bringing up their concerns about my drinking, then there's every chance I would still be drinking (or it might have killed me in the long run).
I think as a general rule, people feel more able to discuss the really socially unacceptable issues (like alcoholism or drug addiction), but being overweight is such a personal issue and there's so much in the media about being 'body positive' and 'loving yourself' that the person who brings it up to someone who is overweight is seen as being shallow or judgmental, rather than concerned or caring.