Lost my virginity to a step sibling technically, my dad and her mom started dating and first night we met and was all together under one roof we just kinda went at each other when our parents walked out the room (making out real fast before they got back). Our parents left to go out to the bar for a bit and before they was probably at the end of the road she had me going down on her (which at that point I had no clue how to do properly) out of frustration with that I think she climbed on top and we proceeded to have sex.

From that point on we was "dating" and our parents knew because she evidently told her mom. Wasn't really that weird until our parents got married. We finally stopped being together as she pursued guys closer to her age or older and I moved on to a more normal relationship myself.