((((kls))))
I'm sorry you have this unsettling in your soul, it's very hard to live with and takes a lot of energy to contain it, energy that I'm sure you could use for better things in your life and those around you. That is the reason that forgiving is a gift to yourself not the other person but to you.
I hope I can explain this to you the way I see it which is this: I believe in comeuppance, that everyone gets their reward for the good or bad things they do to others. If you can in your minds eye look at your sister now in her state of unawares, no quality of life, lonely, and brain dead hasn't she received her comeuppance? What a sad state for someone who swindled other people out of their money. I wouldn't wish that on my worst enemy and I'm sure it gives you no pleasure to see or know the condition she is in now.
Now for your benefit what you can do symbolically is to get all your feelings out and put them in a symbolic box and give her that box, but once you've given it to her you must put the whole thing to rest and never think of it again or speak of it again to yourself or anyone else........that is what it means to let it go.
The above is a guided imagery technique, you might want to have a therapist or someone trained in this kind of work help you with this, but it really can be as easy to do as I've stated above.
You could take it one step further depending on how you feel about this and do the mental and emotional work of placing all your feelings (good bad or otherwise, they are yours it doesn't matter how you feel) in a real box and tie it up with a ribbon and take that to her. In her condition she probably can't open it (it would be empty anyway) and it can just sit by her bedside for others to comment on, only you know the meaning behind the box.
Just some suggestions, but I am real big on forgiving and forgetting and letting it go I like to keep my life uncluttered of negative energies. I hope this helped you with your decision and I wish you peace of heart kls.