FutaMari

Once every year, Sunny and Mari would have a competition of sorts. One to see which one of them was bigger.
The two of them had their fully erect cocks right next to each other. And Mari's was bigger, of course. She couldn't help but grin at Sunny over this fact. "How does it feel to know you'll always be littler, brother?" Sunny's was making an adorable pouty face. He was probably a little upset. He'd finally grown to be almost as tall as she is, but not all of him was catching up. She tried to console him a little by gently petting his head. "There, there. I'm sure it'll happen next year. But until then..." Sunny looked to her, excitement written on his face. Hehe, this really his favorite part.

"Come here, Sunny." Sunny rested on the floor, as she gently guided his head to what was between her legs. She would never admit this to Sunny, but her penis isn't exactly the biggest. Sunny's penis used to be surprisingly small, small and cute just like him. However, it had grown quite a bit in the last few years. He might truly get bigger than his big sister...the thought excited her terribly.
She wouldn't get too much time to contemplate that train of thought as Sunny started getting to work. He began to playfully bat at it with his tongue, teasing her. It always felt like he was baiting her into being rough with him whenever he did this, but she wouldn't. She knew he liked it, but she enjoyed starting things off slowly. Sunny eventually got the hint and began getting to work proper. Tongue licking around the head and resting on her frenulum, and slowly taking it into his mouth as he began working on the shaft. Mari always wondered why he insisted on doing this for her. She could do it for him, too. It's not like she's the only one with a penis here. It felt nice, but she didn't want him to feel neglected, so she started petting his head. "You're doing great, Sunny. I love you so much." She praised him as he kept up the good work.
After a minute of Sunny's expert work, his mouth and tongue deftly moving around the entirety of her penis, she began to reach her limit. "S-Sunny, I'm ah-bout too-" Mari felt him speed up his movements, coaxing her into releasing her first load, Sunny made sure he took it all the way into his throat. Mari could only let out moans and gasps in her pleasure, but made sure to keep petting Sunny's head. She knew how much he loved being pet. After a moment it passed, and she let out a sigh. "That was wonderful, Sunny. You're getting so good at this." Sunny had pulled away and lifted his chin up. She gave him a smile seeing him try to swallow the last of her semen. "Sunny, you don't have to drink all of it. Surely it can't taste that good." Sunny made quite the interesting expression as he gulped down the last of it. "I know...but it's a gift from you. I can't just let it go to waste." Mari giggled. "Sunny, that's silly." She kept petting his head as she helped him up, and next to her on the bed.

Mari and Sunny sat side by side for a moment, Mari stroking his hair lovingly. "Sunny, you're always doing this for me...can't I do something for you?" He looked up at her with his cute face as he gave her one of his rare smiles. "big sis' love is enough." Mari would have to be insane not to pull him into a tight hug after that. "How can you be so precious?!" She practically yelped as she rubbed her face against his hair.
"But seriously...I had an idea I wanted to try." Sunny looked straight up to see her face. "I think you'll like this."

Mari had bought some lubricant and was applying it in Sunny's cute butt. He let out adorable squeaks and other sounds as she did. Part of her was enjoying this a bit more than she should, playing with him. She found herself squeezing his cheeks a little, which made even more cute noises from him. She put a bit more than she needed to but she wanted to be safe and not hurt him in any way. She was trying to make him feel good, after all.
Once she was finished she patted her lap. "Sit here, but facing me." Sunny did as he was told, putting his face really close to hers. She could see him light up in embarrassment. "Alright, I'm going to put it in slowly. Please let me know if it's uncomfortable, or too much." Sunny nodded.

Mari wrapped her arms around Sunny, and held him by his armpits. Once her penis found Sunny's butt, she slowly spread it open and pushed in. His face scrunched up a bit as she took it slowly, inch by inch. It didn't take long before he had been set down on her lap, taking the full length of her inside. "Well done, Sunny. I'm proud of you." Sunny nodded again. "thank you" he whispered. "I'm going to move again." Sunny nodded again. Keeping her hands firmly around Sunny, she began to lift him slowly. She wanted him to acclimate to this feeling. Sunny's face continued to make odd expressions, but he didn't say anything. Eventually she brought him back down. And up. And down. Soon she began to speed up. Sunny was starting to let out more sounds. You see, there was a reason Mari chose this position. By holding him so close to her his penis would be rubbing against her belly, stimulating it and making sure he feels good there, too. Her efforts paid off as she started to see his face make a pleasurable expression. It brought a smile to her own. Only the best for her dear little brother.
"M-Mari..." Sunny moaned out, looking at her. "What is it?" She whispered as she nibbled his ear, feeing his penis twitching against her. "I-I...lo-ah-" He couldn't say it before she felt he let semen spurt onto her belly and lap. Mari couldn't comment on that as it wound up pushing her over the edge, causing her to let out her own inside of Sunny. "O-oh-ah, Sunny-" was all she could say as she felt her penis twitching inside him.

The two of them sat there for a moment, basking in this feeling. Mari pressed Sunny against her chest the way she always does. "I love you, Sunny..." She let out a few words as she cooed into his ear, stroking his head. "Mari?" She heard him whisper, as she pulled her head back to meet his gaze. His eyes were cloudy, but they met hers. "What is it?" She moved her face to meet his, touching noses. "can I...make you happy like that...?" Mari gave him a peck on the lips and squeezed him tighter. "Of course. Next time, though. It's getting a bit late..."
Mari led Sunny to the bathroom, and they took a bath together to clean off what was left. She wanted this moment between them to last longer, so she spent a bit more time cleaning his hair than necessary. She was happy to see his cute little cowlick remain unharmed by her brushing. Sunny seemed exhausted, barely able to keep his eyes open. He fell asleep halfway through and she had to carry him back to their room.
"Goodnight, little brother." She gave him one last kiss on the cheek as she cuddled him, falling asleep soon after...