When Sebastian and Brittney got into the pool Sebastian was disappointed that Brittney brought a one piece bathing suit and not a bikini but still she was very hot. After chit chatting for a while he told her: "Look, Brittney you've got to get even with Kathryn and I can help you."

"What are you talking about Sebastian?"

"I'm talking about my bitch sister hiring a call girl to seduce your boyfriend because you didn't invite her to your party."

"Oh that girl is an obsessive basketball groupie. As much as I despise your sister I don't believe those crazy rumors. In fact I think she started spreading those rumors just to show everyone who's the boss around the school. Anyway, that's irrelevant. I dumped Mark. Besides what's it to you?"

"My sister's enemy is my friend and you are her enemy now." Sebastian inched closer to her, looking deep into her eyes.

"Just like Daniela Eggert?" She asked.

"Just like Daniela Eggert." He replied.

"Aren't you still seeing her?"

"No, not anymore." He said and got as close to her as he could without touching her.

"What the hell is going on here, Sebastian?"

"Look, Brittney, my sister hates your guts and you have a gorgeous body and that makes you irresistible to me." He said and put his hands on her waist.

"Is that why you invited me over here, Sebastian?"

"Come on Brittney, it's Saturday night and I invited you over for a swim, didn't you in the back of your mind think anything could happen?" She opened her mouth to say something but then hesitated and took a deep breath.

"Look, Brittney," He continued, "I'm totally attracted to you and I believe you are attracted to me and If my sister knew you came here to see me, well…"

When Brittney began to smile at him he realized it was a done deal. When they kissed he could feel her tongue reaching deep into him. Getting her swim suit off was a bit awkward but it wasn't the first swim suit he ever took off a girl in that pool. He suggested they got out of the water and do it just outside the pool and since the floor nearby the pool was hard and uncomfortable, he let her be on top.

Several minutes after seven o'clock Mark Hollings was in Kathryn's room. Kathryn told him the rumors about her hiring a call girl to seduce him in order to get back at Brittney for not inviting her to the party were lies.

"I know, Mark said, "That girl was seeing a bunch of basketball players."

"Still," Kathryn said, "Why did you go out on a date with her?"

"I don't know…I just did." They talked for a while and all along Kathryn would gently touch him from time to time. After a while Kathryn said: "You know, Mark, I don't judge you, God knows I haven't been faithful to all my boyfriends. It's just that I've been trying to get your attention for months and you haven't noticed me and then that girl come along and just like that…."

"What do you mean Kathryn?"

She smirked at him and said: "Oh, you know what I mean Mark." She put her hands on his shoulders.

"Kathryn, look…"

"Shhh. Mark, it's Saturday night and I invited you over for a talk, didn't you in the back of your mind think anything could happen?" Kathryn knew at that time that he was getting hard. She was wearing a black mini skirt Sebastian helped her choose, which was just a bit too tight but not too short. Her fingernails were painted red which Sebastian told her would be most seductive. It was all working.

"Look, Mark, I've been fantasizing about your for a while and you're not dating Brittney anymore…" She didn't have to finish that sentence. He was all over her. A couple of minutes later when they were both naked she told him that in her fantasies she was on top. Mark obliged.

Later that night Sebastian knocked on Kathryn's door. When he entered their room they both smirked at each other and realized their plan has worked.

"Got the tape?" Kathryn asked him.

"Sure, you got yours?"

"Yup, let's watch your first."

The camera was pointed to the spot where Sebastian had sex with Brittney and there was no sound.

"Too bad we can't see the whole thing but it's a big pool and we had only one camera available for this."

"Yeah." Sebastian said. "You should have heard the conversation."

"D'you use the didn't you in the back of your mind thing?"

"Sure, worked like a charm."

"Wow she really got her knees scraped, the poor girl."

"I didn't hear any complaints. I guess the pleasure was so intense." They both giggled.

"Yes yes you are irresistible blah blah blah." Kathryn said and laughed. "

Afterwards they watched Kathryn's tape and when they got to the part where Kathryn asked Mark if he didn't in the back of him mind think anything could happen they both laughed.

"Worked like a charm." Kathryn said.

"I told you that skirt would be perfect."

"Oh please he'd all over me no matter what I wore."

"Yes yes you are irresistible blah blah blah." Sebastian said and they both laughed.

"Look the time here", Kathryn said, "Mark and I started having sex at 19:17 and finished at 19:33. You and Brittney started at 19:08 and finished at 19:29. We were doing it at the same time!"

Later they said good night to each other but Sebastian wasn't quite ready to leave her. Soon after he started to walk away towards the door he turned around and grabbed her from behind at her waist.

"That was incredible sis." He said. "These two were girlfriend and boyfriend just a week ago and we…" He kissed her neck and took a deep breath. He could feel that Kathryn was also breathing heavily. "We decided to fuck them." He pulled his hands up and put them on her breasts. "We picked the time, the place, the position. Fuck sis, we can fuck anyone we want!"

Kathryn let out a soft moan. She could feel Sebastian becoming hard. "Yes, Sebastian, anyone we want", she moaned.

"Damn it sis, I have to fuck you now." He said with a deep sigh.

"But it's Saturday. The rules!"

"Yeah, but if we both want it.."

Kathryn sent her arms backwards and grabbed his ass. The success of their plan made her extremely horny. The sex with Mark Hollings was fun but it wasn't nearly as hot as fucking her step brother.

"Brittney wasn't enough for you?" She asked.

"You know that she is just a warm up for you. You know that, don't you sis?"

Kathryn knew that this would be his answer but she wanted to hear him say that.

"New rule", she said and gasped for air. "From now on fucking on the weekend is OK if we both want it."

She turned around and they kissed. Their tongues almost intertwined. Sebastian still had smell of chlorine from the pool on him but it bothered neither of them. Unlike the night before when they were both nervous, this time was much more intense. They didn't exchange a word all through the night and only took a few minutes to get going again after each time they came. In the morning they lay in bed cuddled and exhausted.

"How sick are we?" Kathryn finally said. We have sex with someone else but that only makes us hornier for each other."

"yeah." Sebastian said. "That's the way it is sis and I don't know how this is gonna change. Let's face it, I could never get enough of you."

"And it's not like we can be exclusive or anything like that. If there's someone I like, I've got to have him immediately, even if I always come back to you."

"Yeah. A couple of hours before we got things going there was someone else in this same bed but it didn't feel weird or anything it was simply….awesome."

"It's like having sex with other people only makes what we have here just better and better. The arrangement we have is well, perfect."

"It sure is sis. But I don't get one thing. How is this a revenge for you? I mean her having sex with me is like the most opposite of revenge that I can think of."

"Don't get too full of yourself. I have it all figured out. In a couple of weeks I will throw this huge amazing party. I will approach Brittney and say that I want to bury the hatchet and as a sign of good will I want her to co-host the party with me and since Mark cheated on her he won't be invited. Now I will get credit for doing the honorable mature thing, letting her who didn't invite me to her party co-host my own party so my social status is restored. She will get credit for moving on with her life after Mark cheated on her."

"That sounds interesting but she still hates your guts. Why would she want to co-host a party with you?"

"Here's the really cool part. She'd love the idea of planning a party with me while sleeping with my brother behind my back."

"So you want me to spend some more time with her?"

"You don't mind do you?"

"I'm at your service."

"Good. Maybe at graduation I'll tell her about my little encounter with Mark last night. Now mom and dad will be home in a couple of hours and I want to shower before they arrive. That means we can do it one more time."