Extradite the grandmothers for a start!
I think if she wants to eat, let her eat. Too many people have taken decisions away from her in the past - she needs to learn her own way. She'll soon make the link between eating lots and putting on weight, and decide if that's the path for her or not. You must care an awful lot and the hardest thing would be to do nothing, but the only way she will begin to feel better about herself is to take responsibility for her own life - with your and dad's support - and see what improvements and good things she can do by herself. I'm sure there are many.
As an aside, my mother's on weightwatchers and lost a stone, but it only works if the individual is committed to losing weight, which is ultimately their choice. If she wants to go, why not, but it certainly isn't a long-term solution.
Best of luck.
