Hooking up with my best friend's sister?

I'm extremely attracted to my friends sister. We never get to see each other but whenever we do I can tell she is into me. Last night we seen each other for the first time in months and we all went out clubbing and she couldn't keep her hands off of me all night. The problem is my friend was there as well so I wasn't showing her much interest and I'm scared she might just think I don't like her. Would it really be such a terrible thing to hook up with her just one time? The tension between us when we're together is unbearable and maybe just a one time thing would relieve it for good. What should I do?
Hooking up with my best friend's sister?
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