If it's any help, I work with kids who have learning difficulties and disabilities and have for 13 years. The hardest and most devastating part of the job is seeing how many of them feel worthless and unloved by families who just don't care because they are a burden and unwanted- your granddaughter doesn't have that, she has people who love her so much they're willing to fight for her, trust me, that's the biggest hurdle!
One of the things I always tell my students when they ask why they can't be 'normal' is that another definition of normal is 'mundane' and that means boring, who wants to be boring? They can reach their goal, they just may need the winding path through the countryside instead of taking the motorway (highway)to their destination in life and that's always more interesting anyway!
The support you and her granddad give her is the most important thing in her life and when she's old enough she'll realise that her mom is just being selfish because she's sick, not because she doesn't love her.
As a Pagan I can't tell you that God will reward you for this, can't speak for him :) But I can say that the universe balances all things and selfless acts are returned tenfold, even if the currency is just the love you receive.
You are one of the most positive people I know and your granddaughter is lucky to have you as a role model.