What options do we have? Is this possible?

Ok, I have what I consider to be my little brother (he's 14, about to be 15) in a mess. First of all, let me state that he is in no way legally or blood related. He is just like a little brother to me and has been for many many years (Im 28). Back just before he turned 13, his mother was stabbed to death in a car jacking. Immediately after that, his fathers side of the family tried to get custody of him (none of them had ever been in his life previously because the father had been in prison since he was 10 months old and was still in prison) luckily we were able to stop them and get custody granted to his grandmother (mothers mom) and terminate the fathers rights. But now she is becoming very ill and might not be able to continue caring for the child. Can she transfer custody to me?