Marx/Kamui/Ryouma 3P

 "K-Kamui! Why are you undressing!?"
"Neither of us wants to fight this war, Marx-nii-san. Perhaps the best way to get our people to calm down is to have a child."
"A child...? Wait, are you saying you and I should-"
"Yes. A child born from the Hoshido and Nohr royal families would likely calm the people, no?"
"Even if the mother was raised as a Nohrian royal?"
"I won't allow it!"
"Oh? Are you suggesting that YOU be the father, Ryouma-nii-san? I suppose you both have a point. It's possible that neither side will accept me as one of them, so it might do for me to have a child with both of you."
"Are you crazy!? I'm related to you by blood!"
"And how long has it been since we saw each other? And I've known my entire life that Marx-nii-san wasn't my brother by blood. I'm more han willing to accept both of you as my partners."
Marx and Ryouma both stood there awkwardly as Kamui stared at them, wearing nothing but the headband that was soon discarded as well.
"Are we doing this or not? I can see the boners, you know."

Marx wasn't sure what was more absurd: The fact that his darling adopted sister had stripped naked and insisted that he and her biological brother impregnate her, or the fact that both of them were actually undressing.
Of course, the fact that Ryouma was also undressing likely factored into that. While Marx couldn't fault him for getting an erection at the sight of a beautiful, naked woman, the fact still remained that Kamui was biologically his sister, and Ryouma knew that, even if they had been separated for so long.
"So, one of you can take my pussy, and the other one can take my mouth. Then we can switch."
"Kamui! Where did you learn such language!?"
"From Camilla's books." Kamui stared at him as though he were an idiot.
Camilla. Of course.
"So, who's it going to be?"
"I will..." Ryouma volunteered. "I will take your mouth first."
"Okay then!"
The three of them knelt down, and Kamui looked back at Marx.
"Whenever you're ready, Marx-nii-san." She said, before taking Ryouma into her mouth.

To his credit, Marx didn't look particularly comfortable with the situation either. The only reason Ryouma had volunteered to receive oral sex was because he wasn't sure he was ready for vaginal.
"V-Very well." Marx said quietly as he gripped his cock in one hand, grabbing Kamui's hips with the other. "I'm... going in."
Kamui pulled Ryouma out of her mouth as he did, shuddering.
"Sorry about that, Ryouma-nii-san. I wasn't sure if it would hurt very much, and I didn't want to risk biting you." Kamui smiled up at Ryouma sheepishly.
"Did it hurt?" Marx asked, sounding concerned.
"A little, but it's not bad. Just start slow." Kamui resumed her licking of Ryouma's cock.

Although Ryouma didn't really have any experience in sex, he could tell that Kamui was rather sloppy. It was something of a relief that his sister hadn't been taken advantage of in her time in Nohr.
"Am I doing that well, Ryouma-nii-san?" Kamui laughed when he gave a sigh of relief.
"Y-Yes, Kamui."

It was obvious that Ryouma and Marx were holding back, and that annoyed Kamui. She wasn't made of glass. She could turn into a dragon for gods' sakes!
"You know you can thrust more than once a minute, Marx-nii-san." Kamui said gently.
"I don't want to hurt you, Kamui." Marx sighed.
"I'm not gonna die if there's a little friction."
Well, if that was how it was going to be, maybe she should try a little jealousy. She started swirling around Ryouma's cock.
THAT got a reaction out of Ryouma, who let out a small moan and reached out to pat her head.
"That good, huh?" She grinned up at him.
"Very..." Ryouma gave a small laugh.
That didn't seem to get a strong reaction out of Marx, so she tried moving her hips in a spiral motion. When she heard Marx moan softly, she grinned even harder.

"It seems you may have learned more from those books than we thought, huh?" Ryouma chuckled.
"Yep." Kamui laughed before taking him deeper into her mouth.

It felt wonderful. Perhaps too wonderful.

Ryouma ran his fingers through Kamui's hair and pulled her head closer, pushing his cock even deeper. Unfortunately, it turned out that Kamui, like most people, had a gag reflex and gave a strangled cough.

Ryouma quickly pulled his cock out of her mouth, but Marx was still furious. He lunged at Ryouma... forgetting that he was still inside of Kamui, and only pushed himself in further. Kamui was barely able to cover her mouth to stop herself from crying out in pain. The last thing they wanted was for the others to come running into the room.

"Okay... Maybe we should try something else."

It was clear that Kamui was angry.
"Since it seems neither of you can handle oral and vaginal at the same time, it seems we're going to have to just stick with vaginal."
"Kamui, we really don't have to-" Marx tried to reason with her.
"Yes, we do." Kamui declared. "You're both still rock hard, and I'm very, very horny. We're doing this."
Marx looked at Ryouma.
"So should you go first?"
"Oh, no." Kamui shook her head. "You're both doing it."
Marx stared at her for a moment.

"At the same time!?"

"Yes." Kamui nodded. "It might be good for you two to be closer together."
"Kamui, there's no way this won't hurt you."
"I don't care." Kamui shook her head. "I barely felt a thing before. A little pain might be good."
Marx heard Ryouma snicker a little, and he began to wonder if Father would be angry if he just surrendered to Hoshido right there.
Reluctantly, he and Ryouma got into position. Kamui slowly lowered herself onto Marx while Ryouma readied himself.
"Are you sure this is what you want, Kamui?" Ryouma asked.
"Gods, yes! Just put it in, Ryouma-nii-san."
Ryouma took a deep breath and pushed himself in.

It hurt more than she'd been expecting. Even if she hadn't let out a painful squeak, the wide eyes and the clenched teeth were a dead giveaway.
"Kamui, if it hurts-" Ryouma began to pull out slowly.
"Don't you dare!" Kamui hissed. "Just... Just give me a moment to adjust."
"Kamui..."
"Try living with her." Marx sighed as Kamui tried to get herself into a comfortable position.
"What's that mean?" Kamui protested.
"Once she's set her mind to something, there's no arguing with her."
"That sounds familiar." Ryouma laughed. "I remember Mother saying that it was hard to get her to eat her carrots when she was a baby."

"Don't talk about me like I'm not here!"
"I don't see why we can't talk, Kamui." Marx smiled almost playfully at her. "It's a good way to pass the time while you get "adjusted" to this position."
"Besides, didn't you want the two of us to get along?" Ryouma added.
Kamui glared at both of them before turning to Marx and smashing their lips together.

Kamui was as inexperienced at kissing as she was at sex.
"Try not to make our teeth hit each other next time, if you would please?" Marx groaned when Kamui pulled back momentarily.

The next attempt went much better. Marx discovered that Kamui's mouth was as sweet as he imagined it would be, and Kamui seemed pleased with Marx's own mouth.
Her movements made her slide up and down Ryouma's cock, and it made him shudder.
He tapped Kamui on the shoulder.

"Permission to cut in?" He asked with a sly smile.
Kamui giggled and leaned back, allowing Ryouma a taste of her mouth.
Marx shuddered as his and Ryouma's positions were reversed.

They continued on like that, switching back and forth before Kamui let out a small moan.
"That... That was way better than my fingers." She gasped.

"Much better, Kamui." Marx laughed. "That was "much better" than your fingers."
"Someone's getting confident." Kamui snickered.

Marx gently sighed before his face tightened.
"I think..." He murmured. "I think I'm about to-"
He wasn't able to finish his sentence before he filled Kamui with his seed.

At the same time, Ryouma let out a gasp.
"Both at the same time, huh?" Kamui giggled. "At least this way I'm almost sure to get pregnant."

Marx and Ryouma both slowly pulled out and gently lay Kamui on one of the futons.
"That was... quite the experience, wouldn't you say, Prince Marx?" Ryouma asked as he prepared two other futons.
"I agree, Prince Ryouma, but I would prefer if we didn't get so... close next time." Marx agreed.
"It wasn't that bad!" Kamui protested. "Keep up that talk, and I'll make you kiss each other next time."

The two men laughed and lied down next to their sister.
It wasn't how they'd planned on stopping the war between their nations, but perhaps it was what needed to happen.