When I was younger (18) I met a girl at the family christmas party. Thought she was a friend of the family (Long story, my family does a lot of work with the homeless and drifters were not uncommon). We talked, spent a lot of time together through the meal, gifts, and we even flirted about shopping trips for underwear and the like. After the party games started, we snuck off for a romp in my aunt's room. Was a lot of fun. We talked for a while after that, and it turns out that she was my aunt's new husband's daughter.
We were both mortified, but later decided that no blood was shared, we weren't in it forever, and my aunt and her hubby clearly weren't either, so we continued for a bit, until she moved off for college. We still facebook chat sometimes.