There is an extremely important reason as to why out of everyone, Jen did not want to run into her sister of all people. Olivia was not born a woman. And for a time, she was know as Jen's older brother Oliver.
When he entered college, Oliver spent a lot of time with a few new friends who somehow had helped him discover that he felt much more comfortable as a girl. Jen didn't know the full details, but ever since his sophomore year of college, Oliver had begun to live full time as Olivia.
Now 5 years later, Olivia was here to stay. She had even made the change consistent on all of her documents. Even her college had listed her as female on their billing. Thanks to treatments, she even had her own pair of naturally grown tits.
But the one thing that stayed behind, was her dick. The last remnant of Oliver. Sure the thing was smaller, and practically useless, and her testosterone was being controlled. But she still had it. She whad been saving up for the surgery, but that was still a bit away.
So, now here Jen is, kneeling right beneath her Sister drooling and panting like a bitch in heat, while shaking her ass in delight for having finally finding a cock to suck. She didn't care that it was small or anything like that.
Olivia smirked watching her sister begging her to be allowed to suck her cock. "Oh my, it seems someone is a little cock hungry today isn't she" she says while pushing Jen's head down to the floor with a pantyhose covered foot. She managed to push her down enough so that Jen's huge tits stuck out to her sides and her chin was set down flat on the ground. Jen just stared upwards, specifically underneath her sisters miniskirt at her hairless crotch. Her small dick was fully exposed, being just covered up by her pantyhose. She wanted to suck on it so badly, but Olivia just wasn't letting her yet. Jen let out a series of whimpering moans in complaint, but Olivia just grinned even more at her hopelessly perverted, living cock sleeve of a sister.
She wasn't being cruel though. In fact, she loved Jen very much. Jen was easily her favorite person in the world. She just liked to be dominat with her whenever she got like this. She didn't know why either, but everyone felt the same way about it. When Jen's switch was flipped, every girl she knew wanted to just make Jen into their submissive bitch. Hell, even their own mother was the same way. She once had Jen act as her chair for an entire meal. She had sat right on her face, sticking her mouth up inside of her pussy and just kept it their for 40 minutes when the rest of them had finished eating dinner. Apparently Jen had swollowed 11 orgasms worth of their mother's cum that dinner, and even though her neck and body ached afterward from holding the position for so long, she never said even once that she disliked doing it.
"Alright Jen, I will let you suck my cock, but..." paused Olivia, allowing for the anticipation to build within Jen. "Before I let you do so, you must first make me cum with only pleasuring my ass." This was Olivia's standard "fee" she requested from Jen each and every time. As a girl, having her cock sucked didn't really sit too well in her head. But if Jen made her cum from her ass being pleasured, she could more convincingly lie to herself that her sister was really just sucking on her enormous clitoris.
Now that she had a path to follow, Jen didn't waste any time. The moment Olivia lifted her foot off of Jen's head, jund shot out her tongue and snaked it up underneath her sister's skirt, pulling down her pantyhose and began slipping it in and out her asshole to lube it up. Now, Olivia's asshole was already fairly loose as she usually only ever had anal sex with her ex-boyfriend. So Jen had no issues with penetrating it with her mouth once her tongue had wet it down enough.
Jen then proceeded to face fuck her sister's asshoke, pumping her mouth back and forth, as if it was a cock, fucking her sister. All the while, her tongue was busy exploring the deeper parts of her sister's intestines since she knew for a fact that Olivia had gone out and got fucked last night. Jen didn't know she had until she pushed her face into Olivia's ass and discovered traces of "less than a day old" cum were still present. It didnt take long before she found the "fresh" load and scooped it up to being it back to her mouth so that she could eat it. Thankfully, Olivia was pretty strict with herself about keeping her insides clean. Usually she would even give herself daily enemas just incase she wanted to go out later and get her ass eaten out. So at the moment, she was very clean inside.
It wasn't going to take much longer for Olivia to cum as Jen found that she was extremely good at this. If it was for the prize of sucking a dick, she would do whatever she had to do to the best that she could do it. Now, she wouldn't commit a crime or assult/rape someone, but shy from doing anything illegal, she wouldn't hesitate to do it and would do it with eagerness. Or at least her body would, as her mind was more of a passenger in her body at the moment. Perhaps if she ever mentally became willing as well, would she regain full autonomy of her own body while in her "cock lust" state.
Feeling her orgasm comming, Olivia tapped Jen on her head signaling for her to pull her mouth out of her ass. Jen did so and was rewarded with Olivia offering up her cock. Jen slurped the small dick into her mouth just as Olivia started to cum, oozing her load into Jen's mouth. Jen used her tongue to block her throat, keeping the small puddle of jizz collected in the scoop-like part of her tongue, mixing it with the one she found up Olivia's ass earlier. Once all cum had been expertly milked out of her sister's cock, she began smearing her new collection all over the cock in her mouth so that she could just suck it all off of it once again. Sure she was essentially playing with her food, but why not? It was something she enjoyed doing and Olivia couldn't bring herself to pull out of Jen's mouth. If she were just a bit weaker willed, Jen's mouth would easily have convinced Olivia to never want to have her cock remade into a pussy. Even right now, the thought of never being able to experience this unparalleled talent for cock sucking again was fighting hard to convince her, but Olivia knew what she wanted and would never budge on it, no matter how amazing her sister's cock sucking felt.