This.KB's advice was pretty sound.
Barring that, go ahead and accept her. The sins of the father/grandmother are not the sins of the son/half-aunt. For all you know, your mother and aunts could be blowing things way out of proportion and the grandmother isn't nearly as much of a bitch as they say. Even if she is, that needn't be the case for the half-aunt. She could be a great person who you like a lot and become close friends with.
I'd say accept it and see how shit goes. If she's crazy or a huge bitch, remove and block her. Oh, also, you say you're scared of the potential psycho seeing you? If you have images online with your name attached to them, it's not hard for someone with basic internet know-how to find what you look like if they have your name. That's a silly reason to be worried about not accepting, she already knows enough about you to hunt you down and kill you if she really wants to. You've got nothing to lose, accept and see what happens.