Possession vs. Cronenberg's Dead Ringers - Other similar movies?

Possession to me tells the same story as Dead Ringers. It's the downward spiral of a normal person brought on by the actions of a loved one. Here the "normal" husband who loves his wife follows her descent into madness. In Dead Ringers, one of the identical twins follows his brother's descent into drug abuse and madness. I really find these stories fascinating. Can anyone suggest other movies that portray this?