This story originally appeared in a shorter version under jimjonesjim. Since then I've changed my pen-name and included the addition you'll see at the end.
Enjoy!
Jim woke with a start. He had thought that he was in the house on his own; at least his mum and dad had gone out after lunch, and they had assured him that they would definitely not be back before eight. His sister on the other-hand, was less reliable in her comings and goings. Maggie had an unnerving tendency to suddenly appear. She never gave warning that she'd be back, and she often crept around the place, startling him at the most embarrassing moments.
She was only slightly older than Jim's 18, barely ten months in fact, but Maggie seemed to him, to be much older. It was partly because their parents always treated her in a much more adult fashion, 'She was the older after all' Jim rationalised, but there was something about her that gave everyone the impression that she was at least in her mid-twenties. Jim had only recently started to question this situation, but over the last few weeks he'd thought about little else.
It wasn't so much an obsession, more a powerful interest, and the reason he'd told himself was that it just wasn't fair. However, upon reflection, he decided that he was not just jealous of the way she was treated, he was increasingly becoming aware that Maggie was a really sexy babe. She was a brunette with a red glow to her hair, and her body was to die for. Jim was no expert but he guessed that Maggie's tits were at least c cups, and her ass wiggled so sexily when he watched her around the house. He couldn't fail to notice either as his sister had begun to wear the most provocative clothes recently. Crop-tops which emphasised her tits and showed a wide expanse of delicious midriff; low cut skirts and jeans that, had they been a centimetre lower, would have shown the world his sister's pubes. 'I wonder if she shaves it?' Jim often pondered these days.
It was strange that for years he had just tolerated his older sister, and tried to avoid her as much as possible, and yet now he couldn't stop thinking about her. In fact, one of the reasons he had woken with a start that afternoon, was because he'd had such a vivid dream about Maggie. Before his rude awakening, he had been watching his sister through an upstairs window as she lay naked on the grass of the back yard. He remembered having a huge hard-on, as he watched her stroking her beautiful, size C breasts. He'd not actually seen her tits in real life, at least, not since they'd become real breasts. They had run around naked as little kids with the garden hose spraying, but all that had been innocent childish fun, and anyway she'd been as flat chested as he was back then. No, this dream was new, now Maggie was a fully-grown woman, and had the power to induce a massive erection when ever he thought about her.
Jim listened carefully as the house creaked around him, and then he heard the noise again. He was sure now that it was his sister's bedroom door squeaking. He rose quietly and crept along the passage. On reaching her door he noticed that it wasn't quite closed, in fact it was open enough for him to see across the room to her mirror where Maggie would sit and put on her makeup.
She wasn't sitting there today, but he could see her stretched out on the bed. She was on her front so he couldn't see much, but what he could see was more than enough. There on her bed, Maggie had lain so that her already short skirt had ridden up her thighs. It had slipped up so far that Jim could see the edge of her red panties. He gave a gasp, fortunately not so loud as to be heard, at least he thought not. The reason for his sudden intake of breath, Maggie had her hand inside those panties, and was busily rubbing her unseen pussy for all she was worth. She was obviously so pre-occupied that Jim thought she would never see him standing behind the door, spying on her reflection in the mirror.
As she slipped her hand up and down he quietly released his own throbbing member from his pants, and started to tug on it. This was heavenly. There was his gorgeous sister frigging herself, and he was seeing her at it. The excitement started to overwhelm him, and within minutes Jim felt the familiar pulse of his cock as it expanded still further in preparation for an almighty gush of cum. He looked down at his shiny purple head, with the wide spread of the glans twitching with excitement.
Looking up again to see how Maggie was getting on he couldn't believe his eyes, or his terrible luck. Because there she was standing right in front of him, glaring at him in what he could only imagine was a look of murderous intent.
'What do you think you're doing Jimmy-Boy?' Maggie demanded, although not as angrily as he had at first imagined she would be.
'I-I-I-I' Jim stammered, unable to come up with any reasonable explanation. He was completely tongue tied, along with the fact that his penis had suddenly shrunk to a tenth of its original size.
'What have we here?' jibbed Maggie.
'How the mighty have fallen! I could have sworn that you were fit to burst only a minute ago, and now look at it'
Jim looked down at his limp member and couldn't think of any sort of response. Instead he turned on his heal and headed back to his own room, with his mind racing. What was he going to do now? Maggie was bound to tell his parents and then who knows what the consequences would be. Then he suddenly realised that telling Mum and Dad was the last thing Maggie would do. What was she going to say? 'Jim was wanking off while he watched me play with myself'. Hardly. So what would she do?
His question was answered almost immediately, when with a swift knock on the door, Maggie came into the room. Jim gave a start and was just about to begin an apology when he noticed the look on his sister's face. Rather than looking angry she was apparently quite composed, and he thought he could detect a smile playing round her lips. Maggie spoke first.
'I thought I'd better come and put your mind at rest' she said quietly.
'No, I'm not going to tell on you! And yes, it is because I can't really think of how I'd put it. Besides, I'm not really sure if I mind too much, after all we were both doing the same thing and I did see rather more of you than you saw of me!'
She paused, and then with a grin she continued
'You've got quite a big cock you know!'
Jim felt a red blush spread quickly up his face. No one had ever complimented him on his cock size before, and now unbelievably it was his own sister who was doing it. He could hardly believe that she was actually discussing such a private thing with him. The next thing she said knocked him back even more.
'Was it really looking at me that made him that big? I mean you couldn't really see anything could you?'
Jim nodded dumbly and then shook his head. After another few seconds he found some words.
'Yea, well, I couldn't see much, but I knew what you were doing'.
Maggie smiled,
'... and what do you think I was doing?' she asked, in a low voice that Jim wasn't familiar with.
He shrugged, and went redder still as he tried to find the words.
'I'd really like to know bro. I want to hear you tell me what I was doing.'
Jim studied his sister carefully. She wasn't taking the piss he decided. Rather, she seemed to be enjoying the conversation, and had turned pink as well.
Well things couldn't get any more embarrassing, he decided. So in his next breath, Jim took the bull by the horns and replied,
'You had your hand down your panties, and you were playing with yourself'.
