a demon from Talmudic mythology which briefly entered the body of her great-uncle after he was killed during Mengele's twin experiments at Auschwitz.

Dybbuks are not actually demons; they are wandering dead spirits which possess the living. You can get demons from the other side of a mirror, though. See various stories by Isaac Bashevis Singer for details.

and mirrors are the easiest doorways through which a dybbuk can enter the fleshly universe ("...what is a twin but another sort of mirror?" Grandma remarks, in perhaps the film's best line).

Okay. That is a good line. And you cover all the mirrors in the house where a death has taken place.