Alex Robinson slipped quietly into the bathroom and locked the door. He hoped that Melissa hadn't noticed the erection that was forming the tent in his shorts. It wasn't his fault, Alex reasoned with himself. So what if Melissa was his twin sister. Alex didn't cause her to go swimming in her skimpy bathing suit, which so elegantly displayed her beautiful 18 year-old body. Alex had seen Melissa stepping out of the in-ground swimming pool in their family's backyard. He noticed her firm breasts first, covered only by a black swimsuit top. His eyes quickly became fixated on the crotch of her bikini. Alex realized that his virginal sister had to have shaven her intimate hair before she could have put on that bikini. The image that came with that thought began to give Alex an erection. As Melissa came into the house, Mrs. Robinson commented on her daughter's choice of swimsuit.
"Melissa sweetie, that's fine for around the house, but don't let me catch you wearing that anywhere else, or that swimsuit will be showing off some toasted buns the next time you put it on."
"Yes ma'am."
Melissa walked towards her room to get dressed. Mrs. Robinson "toasted buns" remark had been a veiled threat of a spanking. The Robinson family was a very religious family that believed in traditional discipline. Unfortunately for the parents, but fortunate for the two siblings, the child-abuse laws of their state prohibited the spanking of children. It was some bleeding-heart liberal garbage about spanking having similar psychological effects on children as molestation. The Robinson family knew that the Bible was more accurate in the field of psychology than today's quacks with degrees. Of course, the siblings had recently turned 18, and one of the conditions of them being allowed to continue living with their parents was that they would have to submit to whatever punishments were deemed necessary - including spanking. Neither Alex nor Melissa had been spanked yet, but they knew that if either of them were to be unfortunate enough to earn a spanking, it would be a very unpleasant experience. The threat alone ensured their perfect behavior.
The thought of Melissa getting spanked did it for Alex. He had a full erection now. He quietly walked towards the bathroom as his sister approached from down the hallway. She seemed to briefly glance downward, but showed no signs that she had noticed anything unusual. Once Alex was safely in the bathroom, he sat down on the toilet and took his cock into his hand. He thought about Melissa sitting in the tub, her legs spread wide, her razor in between her legs. Alex felt a twinge of shame as he started stroking his cock. He shouldn't be masturbating over his sister, but yet that was exactly what he was doing. He closed his eyes and imagined that it was not his hand running up and down his cock, but rather Melissa's small soft hand.
Alex heard a faint noise from outside the bathroom door. He made sure to masturbate more slowly so that the sound of his hand sliding on his cock wouldn't be audible from outside of the bathroom. He also made sure not to breath too heavily. The thought of someone, probably Melissa, standing right outside the bathroom door while he masturbated turned Alex on even more. It took some self-control to stifle his orgasmic gasp as his cock erupted.
After cleaning up, Alex opened the bathroom door. Just as he suspected, Melissa was casually leaning against the wall beside the bathroom. She smiled at him and then walked away, her bikini-clad buttocks swaying slightly as she walked. Alex was overcome by an awkward embarrassment. He tried to convince himself that Melissa had no idea what he was doing in the bathroom, or if she did, who he was thinking about while doing it.
Alex felt the sudden urge to hide out of a sense of shame, so he headed towards his room, which was conveniently located next to his sister's room. It just so happened that Melissa's door was slightly open. Alex stopped to watch Melissa pull the waistband of her jeans up over her white cotton panties that she had just put on. Her back was turned to him, so she didn't know he was there. Melissa then removed her swimsuit top. Alex gazed at her bare back, wishing he could see her front side. Alex had just finished masturbating, so he wasn't getting too sexually aroused by this sight, but it did stir his curiosity. He quickly sidestepped out of the line of sight and walked into his room.
Alex sat in his room studying his textbooks. After about an hour, he decided to go to the kitchen to get something to drink. Melissa exited her room at the same time Alex exited his. They immediately made contact and stopped. Melissa smiled at Alex.
"Hi there, brother."