This is just my opinion but I feel that sometimes something else is going on when people see ghosts after someone we know has died. A familiar spirit is just that, completely familiar with a person. I think evil spirits can masquerade as the deceased with the motive of taking advantage of the situation. This is not always the case, but I would be very careful of accepting things at face value when it comes to the supernatural. If your grandmother was never a hurtful person before, I doubt if she would want to hurt you after she died. This could be something evil doing its thing at a time when you are most vulnerable and possibly more accepting since you're thinking it could be your grandmother.