I was sitting on the edge of the bed with my shorts around my ankles, a pair of scissors in my hand and a page printed from the Gillette website explaining how to trim my pubic hair beside me when my stepsister’s voice exclaimed “What the hell are you doing, Tim?”

I jumped and almost cut into the side of my cock.

Ali stood leaning in the doorway with her arms crossed under her boobs. I had thought I was alone. It was Sunday afternoon, the day my parents always went to the store for the week’s groceries. I could usually rely on three hours to myself. Ali should be out with her friends, and I had assumed there was no one home but me.

I could feel my face flushing. “Um…” I said.

“Well?” Ali said.

“I was… going to trim my bush.” I admitted.

“Why?”

“Well… it’s meant to make it look bigger.”

“Make what look bigger?”

“You know,” I said.

“What?”

“My cock,” I said.

“Really?” She looked down at me and laughed, “That would be one hell of trick!”

I was insulted now. “It’s not that small, Ali,” I protested.

“It doesn’t look very big to me,” she said.

“And you know all about cocks, I suppose. Besides, it gets bigger. And you gave me a shock - that’s going to make it shrink too.”

“Huh,” she snorted.

Ali and I were stepsiblings who had turned eighteen this summer -- and very much alike. I was near six feet with dark curly hair. Ali was five feet four with curly blonde locks that fell to her chin, framing her face. She was my stepsister, so I probably wasn’t too good a judge of how hot she was, but if I tried to look at her as just another girl then she was hot. Slim, good legs, nicely toned butt that filled her jeans perfectly, flat stomach and sexy boobs. I had sneaked a look at one of her bras a while back and knew she took a 34B size.

Usually we got on very well together - but maybe not today.

I imagine, like all stepbrothers, I’d had those thoughts about Ali. Most of the guys I hung out with who had stepsisters said they’d had them, and Toby Black told us he’d seen his stepsister masturbating when he peeked into her bedroom one night and it had really turned him on. But it had never occurred to me that I might ever do anything with Ali. My secret thoughts were pleasant fantasies, but that was strictly where they belonged, in the closet marked: “Nice to think about, but no way!”

Ali remained in the doorway with a crooked smile on her pretty lips. I leaned over and grabbed my shorts and started to pull them up.

“Oh no, don’t let me stop you from gaining an extra half an inch,” Ali said. “I’m intrigued now.”

I looked up at her from my awkward position and waited. She made no move from the doorway.

“If I could have some privacy then?” I said.

“Why? I want to watch, see if it does look any bigger when you’ve finished.”

“Ali!” I said.

“It’s you that wants to shave your bush off,” she said. “Go ahead, don’t let me stop you.”

“I don’t want to shave it off. Just trim it a bit.”

Ali pushed off the door and came into the room, grabbed the printed sheet from the bed and dropped into the armchair pushed back against the window. She read the words slowly, looked up at me, then back at the page.

“It says here you can shave them all off if you want. Or shave your balls, there’s a diagram here how to do that. But it does say you can trim as well.” She turned the page to study the drawings. “And shave around the base of it. That’s what makes it look bigger, it says here. OK!” She put the paper down and relaxed back into the seat, resting her chin on her hands. “Go ahead.”

I stared at her but couldn’t think of anything to say.

“What gave you the idea?” she said, putting her head to one side.

I shrugged. “Some of the guys at work were talking. And last week I was showering after my swim and there was this man…” I stopped, embarrassed, hoping Ali didn’t think I made a habit of looking at other men in the showers.

“What about him?”

“Well, he’d shaved everything off. At least it looked that way - I didn’t get too close.”

“And did he look big?”

I nodded.

“Maybe he liked you, Tim. Maybe that’s why he looked big. Did you think of that?”

“It wasn’t anything like that!”

“You like girls then?” she asked.

I think my mouth fell open. “Of course, I do! You didn’t think-”

“No, not really,” she laughed, “but it’s just so easy to tease you, Tim. Go on then, shave those curly bits and let me see how it looks.”

“I told you, I’m not going to shave them off, just tidy it up.”

“So why the razor?”

“To do around the base,” I said. “That’s what it said on the website. Taking a bit off around the base will make it look bigger…”

“Like I said, Tim, good trick - but I’m still waiting for the proof.”

I was getting mad with her now, as I always did when she acted this way - she could be totally infuriating. She sat there with her chin on her hands, watching me, seeming not in the least embarrassed. Okay, I thought, if she wants to watch then let her, see if I care.

I lowered the scissors again and combed some of the dark curly hair up and trimmed the smallest amount off. It didn’t seem too bad, so I worked around some more.

“What about you, Ali, ever thought about a little bush work yourself?”

She laughed. “Don’t be shy, Tim, say what you think!”

I shrugged, then realized it was almost a mistake with scissors so close to my cock.

“Only wondering. It’s pretty common now, or so I understand.”

She smiled. “Let’s just worry about your bush for the moment, yeah?”

Five minutes after I started I was beginning to think that I might have taken a bit too much off, but there was no way to put it back, and I guessed it would grow out soon enough.

I had been concentrating so much - cutting with scissors close to your manhood will do that - I had almost, but not quite, forgotten about Ali. Then I became aware that as I trimmed I was starting to get aroused and my cock thickened until it lay heavily down inside my thighs.

I looked up at Ali and she was watching it with a smile. “It seems to be working, Tim. It definitely looks bigger now.”

“Ha Ha,” I said. “And it might get bigger yet.”

“It’s a good trick,” Ali said.

I was aware of my heart beating in my chest. I was becoming incredibly aroused, and it occurred to me that it was because my stepsister was watching me. To begin with it had felt strange, and then I had forgotten about her and concentrated on what I was doing, but now the idea of trimming my pubic hair was turning me on, and the thought of doing it in front of Ali turned me on even more. I was sick, I thought, sick and perverted - Ali was my sweet stepsister, and I felt bad that my cock was responding like this.

“Sorry,” I said. “Cocks, you know, kind of have a will of their own.”

“Is that right?” Ali said. She was leaning forwards now, her elbows resting on her thighs.

“I guess you already know that,” I said. “You’ve had boyfriends, haven’t you?”

I knew Ali had been out with at least five guys, starting when she was fifteen. I didn’t think there was anyone at the moment, but she had broken up with Doug only three months ago.

“You think I go around looking at cocks all the time, Tim?”

“I don’t know, Ali. Do you?”

She snickered. “No such luck, Tim.”

I laughed. “Ali, you have to be joking. You’re a hot girl. All my friends would do anything to show you their… well, you know…”

Ali laughed back at me, then said, “You really think I’m hot, Tim?”

I glanced across at her. “Of course, you are, Ali… really hot.” The weirdness had just got weirder. I was tossing out compliments to my stepsister while my cock continued to fill with blood and grow hard.

To take my mind of things - as if it was likely to make things any better - I picked up the can of shaving foam and squirted some into my palm then used my fingers to rub it around the base of my shaft.

“What’s that for?” Ali asked.

“I’m going to shave just around here,” I said, pointing to the base of my cock where I had rubbed in the foam. “I think that’s the important bit.”

I used my other hand to move my cock aside. It had filled more and was now maybe five inches long, but still only partly erect. With the razor I started to work around the root, being careful not to remove too much.

“And your balls?” Ali asked.

“Not now,” I said. “Maybe, some time, I’ll see.”

Her eyes were fixed on me as I worked. “I think it would look cool if you shaved your balls,” she whispered.

I swallowed hard and continued to work. It was another five minutes before I finished and wiped the extra foam away with a damp cloth I had brought for that purpose. My cock was now over six inches and had started to lift up from my thigh.

When I looked at Ali her smile had gone, and her lips were slightly parted. If I hadn’t known better I would have said she was getting horny.

“You okay, Ab?” I said.

She gave a little start as though I had surprised her. “Uh, yeah, fine Tim. Fine. It’s really starting to work now, isn’t it?”

“You’re kidding me now, Ali. You know perfectly well what’s happening here.” I was feeling unbelievably excited now, and my cock twitched and filled more. Both Ali and I watched as it lifted clear of my leg and started to stand erect, extending and growing as blood filled the vessels and made it stiff. It was almost eight inches long now, and about as big as it got.

“When will it go down again?” Ali said, color in her cheeks.

I laughed. “You must know about cocks, Ali,” I said. “Didn’t any of your boyfriends get hard?”

“Not like that,” she said. “Didn’t have one who showed me their cock. Didn’t want to see it, to be honest.”

“But you don’t mind looking at mine,” I said.

“That’s different,” she said. “You’re my stepbrother, you’re closer than a boyfriend. We tell each other stuff, don’t we, Tim?”

“We do,” I agreed. We always had been close, that way. But that was that way… not this way.

“So, does it?” She said. “I’m curious. You can tell me anything, Tim, you know that.”

“Does it what, go down again? And does this arrangement work both ways? Are you going to tell me things too?”

“Yeah, will it go down on its own? And I might. What do you want to know?”

“You know what guys do, don’t you? Don’t you do the same kind of thing too, when you get horny? I’ll tell you what I do if you tell me, Ali.” I was turning myself on even more now and didn’t care.

“Tell me what guys do, Tim?” Ali said. “If you tell me what you do, I’ll think about telling you something back.”

I couldn’t tell if she was kidding or not. She must know what I wanted.

“What is it you do?” she asked again. She was definitely flushed now, and was sitting far forward in the chair, her arms hanging down between her legs. It was like she was trying to get as close to me as she could without getting up.

My embarrassment and shyness in front of my stepsister was being swept away by the feelings that ached in my cock and balls.

“I rub it… like this,” I said, and put my hand around the fat middle section and started to stroke it up towards the engorged head and back down.

“And does that make it go down?” Ali said, “It looks to me like it’s making it bigger.”

And she was right. I felt more aroused than I had ever done in my life. The dark head of my cock was huge, purple and engorged, the sharp ridge beneath the head delineating it from the long pale thickness below. Veins showed blue winding along my length, and the central slit at the top was starting to leak clear fluid.

“Eventually,” I said, rubbing a little faster.

“How long does it usually take?” Ali asked. Her voice was soft and husky at the same time.

“That depends.”

“On what?”

“On how excited I am.”