Is it normal that i constantly fantasize about my older sister

I have always had an attraction to my half sister same mother different father. She is hot the perfect woman and very horny. Without getting into too much detail we messed around when we were young and I have had a foursome with her and her husband but no sex with her. I think she is attracted to me as well because when she drinks she kisses and teases me. There is a lot more but you get the picture. I want to have sex with her but don't want to ruin our relationship. We have talked about having sex before but never did it.