Ok. I hope I dont offend you, upset you, or anything else like that ok? These are just my thoughts that are crashing around.
You know my story? So I am going to go at your horrid prob 2 ways.
1. You can refuse to have anything to do with him and that as far as im concerned is your right.
2. You can forgive him. For you. an aside to that would mean you can support your mum through this.
So. number 1 is fairly straight forward I think.
Number 2 however, is not.
As a young girl you delt with this massive problem the only way you could. Without the support of your mum, and I have to say this, but no parent wants to think their dad could of done it, but you should not have been swept aside and not supported. It is a terrible,terrible thing esp for a child to go through.
You have battled through your young life, and everytime you ate, you were adding another layer of support to yourself. Think along the lines of "if im fat, it wont happen.." or something similar.
Everytime you have had something emotionally hard to deal with, you have probably done this same thing. Your emotional coping strategies stopped growing when you were a little girl, it was replaced by the layers of protection you put up against the world and kept on putting up into your adulthood as you didnt know any other way to cope. Its a vicious circle because every time you looked in the mirror, what did you see? Possibly a terrified little girl staring back at you. And so it went on.
You have come to a point in your life where you decided that enough was enough. You got help for your weight, the WLS, and, good woman, you are seing a professional to help with your emotional issues.
Now, something has happened, your abuser, your grandfather, is sick. (He is probably old and may be looking at the door to the next world soon??).
You have come to a roundabout in your life road. You can either go around the roundabout, clean up past issues and swing back and carry on, or you can bypass all that and keep to the straight road.
This brings us to your question: Do I support my family? or do I support myself?
You will never forget what he did to you. That kind of thing stays with us. But you can make it a lesser part of your life. The physical aspect of eating has been "taken care of", now, how bout that little girl? Can she step up and forgive him? She doesnt have to tell him. Just has to "let the pain go", easier said than done, but thats what you have a therapist for

, or, She can walk into that room, look him in the eye and say to him that she forgives him.. She might not actually forgive him, but it might start the process so she can heal.
You do understand that it wasnt your fault aye? A child is an innocent who should never ever be harmed like that. Never. You did what you could do, all you could do, and you were left standing there. If you need to forgive yourself for not doing more, then you must do that. You did all you could.
Right.
So it comes down to the 2 questions at the top.
1. do you walk away? or
2. do you try and forgive and let your mind and soul heal?
I really feel for you being in this position. Its not an easy situation for you.
I really hope I havent crossed any lines or upset you. But you asked and Ive tried to be clear...not sure if I succeeded..
Keep in touch and I hope everything works out for you. And Thank-you for sharing such a tragic part of your life.
*HUGSHUGSHUGS*