I want this girl but its forbidden how do i make her mine?

So I'm really into my girlfriends sister she's beautiful and funny and she flirts with me occasionally I know its wrong but it drives me crazy I already know its a physical attraction that I have towards her and I want to make it a reality I do care about my girlfriend she's always been there for me and we go together great but I just have these same feelings for her sister I want to know if anyone can give me advice on how to test if she really wants something from me or if I'm just gonna throw out my relationship for no reason please I don't need a preacher about how. This is messed up or how I'm an I just want a way to have this beautiful women my mind and body are telling me its what I want so please help