Why not both?

Sam's cock throbbed hard seeing her pregnant sister begging Sam to fuck her. Jenna had offered either her pussy or ass, but Sam was not going to settle for just one. For years she had lusted after her big sister and now she'd finally have the chance to fuck her, so she was not going to miss out on anything. Sure, she had memories of fucking her sister, but those were just that. This would be her first actual time with her sister, and she was going to indulge to her hearts content. She would make sure later to find a way to relive the memories of her first times with her mom and sister, and when she got Jenna pregnant, but for now she was going to fuck her sister senseless.

With a purposeful swagger, Sam climbed onto her sister's bed and lay herself beneath her sister, eager to taste the pussy she had longed to taste more than any other, except for maybe her mother's pussy. Jenna yelped again as Sam's hands grabbed her hips and roughly pulled her wet pussy into her little sister's face. Sam did not wait to go wild with her sister's pussy. From the first lick she attacked the sweet peach aggressively and Jenna moaned hard, barely able to support herself. "Oh god, little sister, you always eat my pussy so good." She screamed. Sam did not respond. She just kept her face buried in Jenna's pussy and feasted to her heart's content. Just as she hoped, Jenna's pussy was absolutely divine. She had tasted many over the years, but none like the formerly forbidden one she was eating now. Sam could not get enough of Jenna's flavor and a wide smile crept over her face as she ate out her sister, knowing she'd be eating this pussy so much more. "She tastes so good. I can't believe I finally get to taste her." Sam thought. All the while that Sam tasted her sister, Jenna's body trembled and she had trouble staying up. Her little sister always made her feel so good.

Jenna was too weak to withstand Sam's oral **** for long, and to Sam's glee Jenna came within a few minutes of her starting. Her face now coated in Jenna's juices, Sam slid out from under Jenna and knelt behind her once more. She licked her lips as she stared at the sight of her sister's pussy and ass, lewdly displayed and ready to be fucked. She had fantasized about this moment so many times and it was still hard to believe she was about to fuck her sister. She reached down to grab her cock and truly felt for the first time how big it was. "No wonder mom and Jenna can't get enough of it." She thought as she lined up her glans with her sister's wet lips. She rubbed her head up and down Jenna's lips and this made her whine. "Stop teasing me, little sister. I'm so horny. I NEED your big cock in my pussy or ass." Jenna begged, shaking and shimmying her treasures for Sam's delight, tempting her sister to take her.

Sam's teasing did not stop though, which frustrated Jenna. She needed to be fucked, and she was so mad that her sister was making her wait. "Please, stop teasing me. I need it so bad, little sister. FUCK ME!!" She begged. This was all Sam needed, and she happily lined herself up and thrust into her sister's warm channel. It felt like heaven around her cock as she slowly drove to the hilt inside her sister. Once her cock was buried all the way inside, which made Jenna swoon, Sam leaned forward until her tits were against Jenna's back and her hands reached around and filled themselves with Jenna's milky orbs. Jenna's pussy squeezed hard around Sam's cock as Sam's fingers toyed with her sister's nipples. Jenna moaned really loud when Sam started her nipple play, and one look with her vision told Sam that her sweet, beautiful sister had very sensitive nipples. It seemed it was possible to make Jenna come just from playing with her nipples. Sam grinned finding this out, her hand never leaving Jenna's tits as she kissed over her sister's neck and ears. "I'm going to fuck your pussy, then I'm going to fuck your ass." Sam said with a tone of authority as she pinched both of Jenna's nipples hard, which did as she hoped and made Jenna come. It was a small one, but it was enhanced by Sam's commanding tone. She was telling Jenna this was not up for debate; Sam was taking both her holes.

For a few moments, Sam did not move. She luxuriated in feeling her cock inside a pussy for the first time, at least the first time she was actually experiencing. That it was her sister, the woman of her dreams, made it even better. Sam let go of Jenna's tits, then rose up again until she was knelt behind her sister and roughly grabbed Jenna's hips. With torturous slowness she withdrew from her sister until only the tip remained before shoving back in. With each stroke she slowly picked up her pace, making Jenna moan with each thrust. It was the best feeling Sam had ever felt, feeling her cock deep inside her own sister. It filled her with more joy knowing this would not be the only time and that fucking Jenna had not just lived up to her fantasy but surpassed it. At this point her thrusts were hard and fast, and Jenna was screaming with each one, nearing the precipice. Sam held out for as long as she could, wanting to come with her sister. Just as she felt as if she could hold no more, she heard a loud scream from Jenna.

"Ohhhh, fuck, YESSSS!!!" Jenna exclaimed as she came, her walls squeezing hard around Sam's prick and that was all it took. Sam's balls tightened and she felt her mind go blank as she shot load after load into Jenna's pregnant pussy. "I can't believe I just came inside my sister; this cannot be real." Sam thought as she launched her last come shot into Jenna's pussy. It was real though, and the best sex Sam had ever experienced. Her new memories even told her that Jenna would never let Sam come anywhere except inside her mouth, pussy or ass, unless it was on her tits.

Jenna was still panting and sweating as Sam pulled out, her cock now covered in their combined juices. The sight got Sam hot and with her new flexibility she easily bent down and took her own cock into her mouth. The taste of her sister's pussy was so good, as was the taste of her own cock and cum. She happily stayed down, giving herself a nice blowjob, slurping and sucking noisily. Jenna turned her head back to see her little sister sucking herself off and it got her wet again. "Fuck, I love when you suck your own cock, Sam." She moaned. Sam smiled hearing this, knowing that she'd done this before for her hot sister.

As she cleaned off the last of their juices from her cock, Sam let her member out of her mouth with a loud pop. She really wanted to finish and swallow a load of her own cum, but the desire for that was overpowered by her need to fuck her pregnant sister's ass. How could she deny herself the chance to indulge in so many of her kinks at once? Pregnancy, ****, futanari, and anal all at the same time. It was like something out of her favorite erotica and now she was living it. Before Jenna could voice her next thought, Sam dove her face into her sister's ass and began to rim her. She wanted her sister's ass nice and ready for her hard cock and she knew this would do the trick. As she licked, the tangy taste of her sister's ass was starting to mix with the taste of her sister's pussy, showing Sam that just as she had written, women's asses now lubricated themselves with the same juices that lubed their pussies. As she continued to eat her sister's ass, Jenna moaned deeply from the feeling of her little sister's tongue on her rear hole. Sam herself was in paradise, worshipping her absolute favorite ass. She was still in disbelief that she was about to fuck her sister's ass. How many nights had she dreamed of this, and now it was real. At this moment, Sam thanked every **** above that she had found the Rulebook and had the courage to peer inside.

"Little sister, please, I'm ready. Please, fuck my ass." Jenna begged. Those words were music to Sam's ears, and she happily rose up again and lined her cock with Jenna's rosebud. Jenna's ass was very wet and loose now, ready for Sam's hard cock. She stroked it along her sister's crack a few times before pushing it against her anus. To Sam's shock, Jenna's ass easily accepted the huge member. Not as quickly as her pussy had, but close. It was still very tight inside her sister's ass but also very pleasingly wet. Sam smiled wickedly as inch after inch of her prick disappeared into Jenna's beautiful ass. Jenna hummed and moaned the whole time until Sam was balls deep in her big sister's butt.

"Please don't wait, just fuck my ass." Jenna begged and that was all Sam needed to hear. Unlike earlier, her thrusts were not slow. With wild abandon Sam thrust hard into her sister's backdoor. Her hands held tight on Jenna's hips for grip, while wet slapping sounds of flesh on flesh filled the room as Sam reamed her sister's ass. It was the greatest feeling in the world to Sam and she had the Rulebook to thank. She could barely hold on and knew she'd come soon, and the next words from Jenna sped that up.

"I'm going to come. I'm going to come. Fuck, I love when you fuck my ass, little sis." She mewled. Those words triggered something deep in Sam and with a hard thrust and a heavy grunt she hilted inside Jenna and came deep inside her sister's ass. Jenna felt her sister's cum coat her insides and like always it triggered her own orgasm. Jenna never told anyone how much she loved anal, but at this moment her little sister knew, all thanks to her new gifts. For the next few moments, the two sisters were oblivious to the world, before both regained their senses. With a slow resignation, Sam pulled her cock from her sister's butt and crawled up the bed before pulling Jenna into her arms.

As soon as Jenna's head lay upon Sam's breasts, her mouth sought out Sam's nipple and happily sealed around it before she sucked hard. Sam could feel her milk flow from her breast into her sister's mouth and she just stroked Jenna's hair as her big sister drank her milk. Jenna drank for a few minutes before letting Sam's tit out of her mouth and opening her eyes, looking up at her sister with love and joy. "That was just what I needed, sis. Thank you." She said before she put her lips to Sam and the two started to share a slow, but romantic, kiss. It was the kind of afterglow cuddle kisses that lovers shared, and it felt amazing for Sam. She had just fucked her sister's pussy and ass, the same sister she had gotten pregnant. It was almost too much for Sam to believe. She really did wonder if this was all a dream and she prayed it was not.

Sam whined when she felt Jenna break the kiss, but she stopped her whine when she opened her eyes and saw the content, well-loved smile on her sister's face. "Thank you so much, little sis. I really needed that. I love you." Jenna said sweetly, and it made Sam's heart flutter. "I love you too, big sis." Sam said before resuming her kiss with Jenna. For a long while, the two sisters were lost in their kiss and cuddle, oblivious to the world. It was only the voice of their mother that broke their reverie and brought them back. "Girls, dinner." They heard their mom call from downstairs. Both girls broke the kiss and looked towards the bedroom door as they heard this, knowing they had to leave the room. They shared a cheeky grin before Sam rose from the bed and then hefted her pregnant sister off the bed. They headed downstairs hand in hand, and both still naked.