It Was A Bet

I am 19 years old and female when my parents were away I was left at home with my 15 year old brother, I decided to get some of my girl friends round for drinks and weed we were drinking quite heavily and one of my friends had brought round some really good weed.
The upshot was that we were to say the least very out of it and being a bit reckless and silly. I can't even remember how it started but somehow I had been bet that I would or could not seduce my brother. Now I knew that would be an easy bet to win as I knew that my brother often 'checked me out' whenever I was wearing anything even remotely revealing and probably because of the state i was in I took the bet.
Having previously agreed that I would call my friend Sandra's phone if I succeeded so they could listen in to me I went up to my brothers room (he was keeping out of the way because a couple of my friends can be a bit snarky to him). I told him that I needed a break from all the girl talk and sat on his bed with him. Anyway to cut a long story short I let him see a bit of flesh and flirted outrageously with him before suggesting that I wouldn't be adverse to getting a bit physical with him within bounds.
He immediately thought I was pranking him so to convince him I pulled my top up so he could see my boobs and put his hand on them. He began fondling them and then to my surprise went in for a kiss still don't know why but I kissed him back and then hit the call button on my phone. Well things escalated very quickly and a lot faster and further than I intended and he had pushed my panties aside and was inside me. To my shame I didn't even try to stop him and responded to him instead.
That was how Sandra found us when instead of just listening on her phone she walked into his room, my brother froze but by then I had other ideas and told him to just keep going. My brother finished inside me with my best friend standing and watching.