Truth Or Dare??

This isn't actually my family, but this story belongs here. I work 5 days a week, so on the weekends I like to hang out with my best friend, Dave. We mostly hang out at his place because my parents are home. We're both taking advantage of being able to live at home with our parents for the savings mostly. His parents are rarely home, and they're also cool with us smoking or drinking. Most of the time, it's just us hanging out, alone, but occasionally his hot slutty sister Annie is home and she usually joins us when that happens. I say she's slutty, because she fucked me months ago when Dave was taking a shower. She also supposedly had a boyfriend at the time. She was far from the typical 18y.o in experience. I could just tell she was banging as often as she could. It was fast and quiet and I didn't tell Dave for days. He actually wasn't as surprised or shocked as I thought he'd be. He just told me she's getting a bad reputation, and will probably end up pregnant. He said that she got lucky with her good looks and takes advantage of them.
I said it was her looks that trapped me and I wish I was him because he got to see her daily. He admitted that was really hard to not look with some of her sexy outfits, despite it being his sister. I then said she would probably make your grandfather hard. As I laughed. He said that he's had to go have a wank at times because he looked too much. I said, so you'd fuck her if she wasn't your sister? He didn't hesitate in saying he definitely would. The next few times she hung out with us nothing much happened, just plenty of perving. I kept talking about her though. Now that I had fucked her, I was obsessed. She showed disappointment in my cock size though (I'm just normal, I thought), saying she only fucked me because she thought I was bigger. I knew Dave was bigger than me, and I just kept talking about her. I could tell he was turned on by my talk, so I kept it up. The last weekend I went over Dave's as usual, she wasn't there at first as she was out drinking. When she came home late we we still up and had been talking about her. She joined us and was in a great mood, but drunk. She's 18, so legally allowed to drink. Dave is 19 & I'm 20. I started a game of truth or dare that was twisted around to anything sexual fast. It went from all "Truth" questions that got more and more inappropriate, to all "Dares" that were just as inappropriate. My deviant mind, all my talk with Dave, and his slutty sister drunk, began to head in a new direction. It was clear we were steering the game towards them fucking. It was my turn to ask her, she took the dare. I went all in, saying, I dare you to lick Dave's cock. She looks at him and said it's only a game and a dare is a dare. As she moved towards him clearly intending on doing the dare. I was amazed that he wasn't rock hard when he pulled it out for her to lick. He was only half mast. She wasted no time, grabbed his cock and licked the head quickly, just once. I said that not his cock, that's just the tip. Lick along his shaft. She did it straight away saying, My turn! Truth or dare? This was to me. I said, Dare. She said, I dare you to show who is, harder. I look at him as we both revealed our cocks. We were both hard. She grabbed my cock and squeezed for a second, then his, just as quickly, and said you're both hard, but Dave is bigger. Now she had his turn, Truth or dare? she said to Dave. He said, Dare! She said, I dare you to lick my pussy, as she took off her pants and underwear. He said, ok, but it's just a game. The she layed back with her legs open and he began licking her wet pussy. Immediately she's grabbing his head, grinding into his face while moaning. Please fuck me? She asked him. This wasn't a dare, the game was over Dave slid his big cock deep into his sisters wet pussy, and gave her a pounding that was so hard the both came together within about two minutes. Dave nutted in Annie hard. As they caught their breath she said, that was only in the game, everyone promise to keep this a secret? We agreed.
I asked her if I could fuck her next, and she snapped at me, Fuck off! Do you think I'm a slut or something?
I said, you could have just said no. I was still in shock with what I had just seen. I didn't care she was saying no that much.