He feigns ignorance

"I don't know what you're talking about, Buttslut," Chris said, "I'm not doing anything. You're the one who came in here and gave yourself a wedgie. Or do you really think that I somehow made you do that?"

"But... Buttslut..." Kirsty stammered, still laying on the floor in exhaustion. She wasn't sure what to say to that. There was no way she would do something like that to herself, right? And feel that way about humiliating herself in front of her brother? He had to be doing something to her... but what would let him be able to do something like that to her?

She slowly got up from the floor, watching as Chris lifted the stack of papers he was holding from his crotch and looked through them again. He seemed to notice something, and took his pen and started writing. Kirsty realized that the idea of her brother having the ability to control her actions like that was completely unbelievable to her, even in her head, so she dismissed the possibility of Chris' involvement from her mind entirely. Even though he was acting suspiciously, there had to be another, more logical explanation for her own odd behavior.

"Nevermind," Kirsty said, mentally chastising herself for even thinking of something so ridiculous, "Buttslut is going to Buttslut's room. You better not tell anyone what you just saw, or you're dead. Got it?"

"Alright, alright," Chris said with a smirk, turning back to his desk, "Just be sure to close the door on your way out."

Kirsty hurried out of Chris' room and made her way down the hall to her own. She then closed the door and sat down on her bed, trying to figure out what compelled her to do something so humiliating in front of her brother. Even just thinking about the incident gave her a small pulse of pleasure. She still wasn't sure what was going on, but she knew something was still very wrong.

Kirsty thought about what could be responsible for what happened, and realized that she forgot to put her buttplug in this morning. Kirsty had a rare condition where if her asshole didn't have something filling it up inside, she would start getting overwhelmed with a strong desire to masturbate and humiliate herself in front of other people, which would get more out of control the longer her asshole was empty, until she couldn't think of anything else but getting herself off.

Kirsty gasped, and hastily grabbed the pink buttplug off of her bedside table. How could she have forgotten something so important? She had to hurry before her condition got any worse! She started looking for the bottle of anal lube she keeps in her desk drawer, but running out of time, she decided it would be quicker to lubricate it with her own pussy juices. She yanked her panties down to her ankles and plunged the plug in her still-wet pussy, stifling a moan as it rubbed against her clit. After a few thrusts to get a good amount of juice on the plug, she pulled it out of her pussy with a small pop. She lined the plug up with her asshole, then took a deep breath and pushed it in. It took a bit of effort, but she was able to get the buttplug firmly into her ass within a minute.

Kirsty breathed a sigh of relief at the familiar feeling of having something tightly packed in her butt. Her feelings of unusual arousal were already starting to die down. She hated how she was only able to feel arousal in the lust-crazed state her condition put her in, and ever since she found out about her condition she tried to keep it as under control as possible. In fact, the only people other than herself who knew about it were her mother and her doctor.

She couldn't help but think about the closest she came to getting caught by someone else before today. She was home alone and had just used the bathroom, and she decided to leave the buttplug out for a little bit to give her ass a break. However, she started watching TV and quickly lost track of the time. Her mom came home to find Kirsty furiously masturbating on the living room floor, consumed by her own lust. Her mom had to shove the plug back in her own daughter's asshole while Kirsty kept shouting about how badly she wanted her 'mommy' to punish her. It wasn't even ten minutes after that before her brother walked in the front door.

She shuddered just thinking about how close her brother came to finding out about her condition again. It was humiliating enough that everyone, including her own mouth, called her Buttslut. If people were to find out about this... she didn't even want to think of how mortifying that would be.

She put on a clean pair of panties and pants, then headed downstairs for breakfast, hoping that her perverted brother didn't get too curious about the display she put on for him this morning and start snooping. What she didn't know was that Chris already knew about her condition, and had swapped out her buttplug with a identical looking remote-controlled vibrator. And of course, he had the remote with him right now.