This story seems to be growing. Thanks for the suggestions, although I doubt it'll be as long as some of you seem to want!
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I was right about being worn out. After a week of fucking both my sisters at least once, sometimes two or three times, a day I was starting to feel like the living dead. The eighth night after I'd deflowered Pam I went to my room and collapsed on the bed and tried to get some sleep. After a short while my sisters came into my room and looked at me expectantly.
"Not tonight girls." I almost croaked. "I need some rest. The two of you are too much every night."
They both pouted.
"All right, I suppose." Pip said with bad grace. "But Pam was going to ask for something different."
"Like what?"
"Like take us both in the butt!" Pam burst out.
"Fuck you both in the ass?" I was a bit surprised at the sudden suggestion, but intrigued as well. "Maybe tomorrow." I sighed. "But only maybe."
Pam's eyes glittered the way that Pip's did when she had a sneaky plan.
"How about if we promise to do something for you? Would that make it a 'certainly' tomorrow?"
"What are you suggesting?" I asked, intrigued now.
"We wear our old school uniforms all day." My eyes widened and my cock, even in its tired state, twitched.
Pam giggled.
"See. I knew that would work." She said to Pip.
"How?" I asked.
"She peeked at your internet history Pete." Pippa said rolling her eyes.
"Snitch." Pamela laughed.
I smiled too, my hand moving towards my tired dick as it stiffened at the thought of my two beautiful sisters back in their school outfits after a year.
"None of that." Pip said, grabbing my wrist. "We want you all rested for tomorrow."
"And how are you proposing to stop me?" I grinned.
Pam grabbed my other arm.
"Easy." She said. There were a pair of metallic clicks and my arms were suddenly handcuffed to the headboard by the little minxes.
"It seems uniforms aren't your only kink." Pip winked at me. "Now get some sleep. You're going to need all your energy tomorrow little brother." With that they giggled and left me there.
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I finally got to sleep despite my raging hard-on and woke up at about nine the next morning. I could hear the girls moving around downstairs, but they didn't seem to be in any hurry to come and release me. As I lay waiting I began to wonder about the sudden curiosity in anal sex. I had thought about mentioning it myself a couple of times, but hadn't because I didn't think they would be interested. Well, it looked like I had been wrong.
Just after ten Pippa came to let me loose. I smiled as I saw her in her school uniform again. It was a bit tight now, especially across the bosom, and the skirt was so short it revealed that she was wearing stockings not tights. The shoes she had on would never have been allowed in school though.
"I hope you're rested," She said. "Because you're going to need to be for what Pam and I have planned."
"And that is?" I asked.
"You'll find out. Now go and shower and then Pammy is doing you some breakfast." She smiled. "Go! It's my turn to do the laundry and I need your sheets."
I went down to the kitchen after my shower wearing only a bathrobe. Pamela was stood at the table, idly flicking through some magazine as some bacon was grilling. She too was wearing her old school uniform as promised. The only difference to Pip was Pam was wearing long white socks and flat shoes.
"Morning sis." I said.
"Morning sleepyhead." She grinned as she looked up. "Sit down, your bacon is nearly done. Coffee?"
"Please." I sat down as she got my breakfast, a rare event, generally we got our own. I couldn't help but notice how sexily she moved now. In a little over a week Pam had gone from practically hiding her looks to almost flaunting them. Now, just looking at her or Pip was enough to get me at least semi-hard. I smiled to myself as my dick stiffened beneath the bathrobe as if to prove my theory. Pamela placed a cup of coffee and a plate with a bacon sandwich on it in front of me as Pip struggled past with the laundry, taking it out to the washer in the garage.
"Tuck in." Pam said. "You're going to need all your strength today with our little scheme."
"And that is?" I asked with a mouth full of bacon buttie.
"Oh, you'll find out." She laughed with the devilish twinkle that I normally saw in Pip's eyes.
"Pam, can I ask about this sudden interest in butt sex?" I asked, sipping my coffee.
"Well, I asked Pip if you had ever done her up the bum. When she said 'no' I asked if I could do it first."
"Why though?" I was still puzzled.
"Because the last time I was first at anything with you two was when we were born."
"It seems a bit extreme Pam." I said dubiously. "Anal sex looks as if it could be painful to start with when I've seen it."
"I've thought of that." She said brightly. "I've had Pip's vibrator in my butt since last night to loosen up the hole."
"What about Pip?"
"She said she wanted you loosen her up personally."
"How?"
"Don't know. With your fingers maybe?"
"We'll see I suppose."
As I finished my sandwich and my coffee Pam leant across me to pick up the plate and the mug so she could wash them. She glanced down into my lap.
"I see someone's ready to go." She grinned.
I looked down and saw my knob was pushing through the folds of the bathrobe. Blushing for some reason, I covered up.
"There's no need to hide it." Pam laughed. "I used to be scared of your dick you know? It was what made you different from me and Pip."
"And now?"
"Now I think it's the most wonderful thing in the world and I can't get enough of it."
