(Based on reader comments, this story's been rewritten and now includes an epilogue)
I was finishing up my run when I heard voices coming from out back yard. I opened the gate and saw my brother Stevie in the pool with his two friends, Karl and Mickey. I'm the older sister, but not by much: we're "Irish twins" -- Stevie just turned 18 and I'm still a month short of my 19th birthday, and we'd just graduated high school together. Karl and Mickey were closer to my age than to Stevie's.
"Hey," Stevie shouted to me. "Mom and Dad just took off for the day, and they left us money for pizza for dinner."
"Cool. I may join you in the pool in a few minutes. I sweated up a storm just now."
"Um... I should warn you, sis, we didn't change into bathing suits."
"Ewww. The three of you are swimming naked on our pool together?"
"God. no. I mean we just stripped down to our underwear."
"Okay, that's no big deal." I'd caught Stevie in his underwear plenty of times, and I'd dated Karl for over a year during high school. We'd never "gone all the way" -- which is probably why he broke up with me -- but we'd done pretty much everything else, so I was certainly no stranger to seeing him in his underwear.
"Strip down." Karl started to chant. "Strip down, strip down." Mickey quickly joined the chant -- Mickey was always sort of Karl's sidekick -- and Stevie looked a little uncomfortable and told them to knock it off.
"Not likely," I said, walking into the house. Especially since I was wearing thong panties under my shorts and no bra under my shirt.
I came out a few minutes later wearing one of my nicer bikinis. It wasn't skimpy by any means, but I know it showed off my petite figure nicely. It wasn't a suit I'd normally have worn swimming in the back yard with my brother, but I guess part of me wanted to show Karl what he'd given up.
I dove into the water, and for a while everybody swam about separately. Then somebody blew up a beachball that was sitting on the side of the pool, and we all started tossing it around.
Somehow, a volleyball game evolved into dodgeball. As we were dashing about in the water trying to avoid one another, Karl managed to brush his arm against my breasts. I ignored it, writing it off as an accident, but a few minutes later he came up behind me and I could feel his penis, not particularly restrained by his wet boxer shorts, pushing against my ass.
"Stop it," I said to him under his breath.
"Stop WHAT?," he asked innocently.
"Stop trying to cop feels," I said. "That ship has sailed."
"Maybe I'm just taking care of unfinished business."
"We don't HAVE any unfinished business."
"I think we do, you tease."
I began to swim away from him, and he called out, "Hey, Steve!"
"Yeah?"
"Have you ever seen your sister's tits?"
"What? Of course not."
"That's a shame," Karl said, "I have, and they're very nice. A little small, but nice and firm. Big nipples."
"Knock it off," Stevie said angrily.
Karl came up behind me again and before I knew what was happening, he'd grabbed my bikini top with both hands and yanked it over my head, exposing my breasts. "What do you think?" he said, holding onto my arms to keep me from covering up.
Mickey held Stevie back, but he wasn't sure about all this. "Karl," he said, "let's not do anything that's gonna get us in trouble."
Karl led me out of the pool -- I tried breaking away, but he twisted my arm so hard, I almost started crying -- and pushed me down onto the grass, Then he straddled me. "Please," I said. "Please don't. I'm still a virgin."
He used my bikini top to tie my wrists together over my head. I'd be able to break free, but not quickly enough to make a run for it. "Don't worry," her said. "I'm not anxious to go to jail for rape. I'm not going to do anything to you unless you ask me to. Mickey," he called out, "bring Steve over here."
While Mickey was pushing Stevie over to us, Karl yanked down my bikini panties,leaving me naked on the grass. "What do you think, Steve?" Karl asked when my brother came close. "Your sister's pretty hot, huh?"
"Fuck you," Stevie said, trying to free himself from Mickey's hold.
"No," Karl said, "You fuck her."
"Are you crazy?" I shouted, trying to untangle my wrists from the bathing suit bra. Karl grabbed my wrists and put his full weight on them.
"I want to see you fuck your sister," Karl told Stevie. "Then I'll let both of you go."
"No way," Stevie said.
Karl took his hands off my wrists and stroked my nipples. I shivered, despite my anger and frustration. "Then I will," he said. "You don't have a lot of options here: Mickey can hold you for as long as he needs to. Long enough for me to take your sister's cherry." I could feel his hard cock pressing into my belly through his boxers. "But I'm going to be nice here, and leave it up to you. What do you say?" he asked me. "Me or your brother?"