Can't remember name of a movie. Help?

I am looking for the title of a movie I watched when I was approx. 16 years old (I am 37 now). It is about a boy who was raised as a girl. This doesn't come out until the end. The family lives in a mansion and "a boy" is killing people at the mansion. Nobody suspects it's the girl. She is "changing" into the boy and killing people. The movie was probably made in the mid to late 80's. Does anybody remember this movie?