I think you have a mild form of separation anxiety, you feel like you need the protection of your grandmother because you associate your mother's home as an unsafe place to be. Other places are more neutral, so that's why you have no problem sleeping there. In my (non-professional) opinion, a little bit of counselling wouldn't go amiss, to help you identify and rationalise your specific fears, and to help you deal with them.
As for being afraid of the dark... I'm absolutely terrified of the dark, it freaks the bejeezus out of me. Again, its a paranoia and insecurity thing, and possibly being afraid of things that you can't see. A nightlight might help.