Hi,
I understand your worry about your mother's weight, but I agree with the other post, not to nag your mother about it. I am sure she understands the risk of being overweight, and maybe made an internal decision to stop dieting because maybe she would gain back double or triple. I myself have this problem and it can get really frustrating, so what I did was, I stopped "dieting". Now, however, because I am still overweight, and do and always have understood the health risks, I am slowly trying to do a routine. I joined a swim class, provided with my college PE requirement, and it's helping and I am also walking again. I am doing this for myself, and for the fact that I am not dieting, but changing my behaviors.
I know that your mom understands what's happening and what could happen if she decides to not "diet", but it is her decision, and as long as she is not constantly complaining or constantly going in and out of hospitals then let her be. But as sad as this is to say, sometimes it takes a life threating thing for a person to "get it together", I am not one of those people, but I surely can be which is why I am trying to do something before things like my knees, and tiredness, and breathing gets to far out of hand.
Maybe without nagging, just sit down and ask her honestly about constantly dieting and what has been the ups and downs for her. Then maybe you will get some insight as to how she feels about her situation.
Getting her point of view may help you out and to understand that "dieting" is not as easy as it sounds and maybe your mom has found a peace within herself and if so, then good for her. Just love her as a daughter and she as your mom, no matter what.
I know this was winded, but I do hope it helps out.
Stella