Been Hooked Up with My Younger Sister's Friend

She’s twenty-five and a close friend of my sister. One night, after too much wine at her place, she let me see her naked, just to be honest. I didn't touch her or anything like that, but it was clear from the way she looked back, we both knew what this could mean. We talked about it later and agreed it would never happen again, but now every time she’s around, I can’t help feeling drawn in. It's not fair to anyone involved. I just wish these feelings weren't there.