I met my step-sister when I was a wee lad (maybe age 12 or so). My mom has been dating her dad for the last 10 years, but I didn't see her too much since she lived with her mother. Growing up and going through puberty, I had a huge crush on her. I'd stop thinking about it after I wouldn't see her for a while, and got on with my life. But at age 18 the feelings started coming back after a family vacation which included her and her friend. One night of heavy partying led to me, my hot sorta step-sister and her cute friend winding up in the same bed. Some light kissing/touching. Bad hangovers the next day, but now there was a something between us. Eventually it fizzled out as the vacation came to an end and we went our separate ways. Fast forward to today; college break and being near each other again. Despite having a girlfriend on the same level of attractiveness, I can't stop thinking about banging my now soon to be step-sister. This shit has been going on for 10 years. The urge is driving me nuts. Should I go for it?

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tl;dr My soon to be step-sister's smoking hot body taunts me, and I want to smash. Thoughts, feelings, opinions?