Stepdaughter or daughter. Its pretty much the same. Without knowing you, I couldn't say why you feel that way, but if it were me, I would find it very disturbing. At nineteen, she is a young adult, and should have her privacy.

Seriously, it might be a good idea to find out why you are having those feelings by seeing a therapist. I doubt it will shock them. They have heard worse. Get some guidance before you wind up acting on your feelings.

Sam2