Is it normal to like fiances sister

I am 25 my fiance is 22 going to be 23, she has a sister who is a few years younger, and when I am by my self I masturbate to the thought of either just her sister and me, or all three of us together with my fiance telling me to have sex with her and her sister. And at the same time video tape it. I don't want to tell my fiance cause I don't know how she would react but it turns me on so much to think of it. Its not like her sister is ugly, she is by far that. Neither is my fiance they are both hot as h@ll. Is this normal. And what should I do