"Well, could you get small?" Tanjiro tentatively asked. "Like when you are riding in the box?"
Nezuko raised an eyebrow but nodded. After a moment, Tanjiro could see her body start to shrink. Her features became not only smaller but younger, her breasts disappeared, her butt became smaller, as well as her facial structure.
"Like this?" Nezuko asked, looking up at her brother. Even her voice had changed with the new body. It was higher pitched.
Her answer came in the form of Tanjiro's throbbing erection that was now level with Nezuko's face.
She smirked and reached out a small hand, grabbing the head of his member.
"You look so much bigger like this…" Nezuko commented as she began to stroke him.
Laying on her stomach at the edge of the bed she scooted forward. She leaned forward and kissed the tip of Tanjiro's cock, sending a pleasurable chill up his spine.
Nezuko's small tongue began to work the glans, wrapping around his engorged member. She tried to take it deep, but her small mouth could hardly accommodate the head.
Realizing this form's limitations, she decided to just suckle on the head and slowly stroke the shaft with one hand. At the same time, her free hand reached out and gently grabbed his balls.
Tanjiro jumped in surprise as he felt her hand cup him, before starting to twirl her fingers and massage his testicles.
Another moment and Nezuko hummed in surprise as she felt something hot and salty splash against her tongue. She took his dick out of her mouth to examine and found there was a small bead of clear liquid dripping from his tip. She licked it off and tasted that same salty taste.
"What's this?" Nezuko asked, looking up to her brother.
Tanjiro didn't have a good answer.
"It means you are doing a good job," Tanjiro said.
Nezuko smiled and went back to work, suckling on the head.
It didn't take long for Tanjiro to start to feel an orgasm build up. But it was too early to blow.
"W-wait," Tanjiro said, putting a hand on Nezuko's head.
"Hmm?" Nezuko hummed, looking up at her brother and slowly taking his dick out of her mouth. "What's wrong?"
"Nothing just…." Tanjiro pulls back a bit. He walks around the bed before sitting down on the edge and scooting back to the middle of the bed, his head resting on the pillow. "I want to try something else."
Nezuko hungrily eyed Tanjiro's member as it swayed, sticking straight up in the air. She crawled towards him, before throwing a leg over his body, and straddling him.
Tanjiro, looking down, could see Nezuko's small body in all its perfection. Her small pink mosquito bite nipples, the tiny slit between her legs that radiated heat. Even the more subtle things, like her slim tummy, her small shoulders, her petite hips.
Nezuko reached down, grabbed the tip of Tanjiro's member, and lined it up with her nether lips. As the head of his cock met Nezuko's lips, he could feel the intense heat, the incredibly soft flesh, and as she put her weight down, the tip pushed past the outer lips, and he could feel the slimy wet inside start to engulf his member.
"Urg!" Nezuko moaned as she began to try to insert her brother. "It's…. Big. It kind of hurts."
"Oh! We don't have to" Tanjiro started, his brotherly instinct to protect her taking over for the moment.
"No, I want it," Nezuko confirmed.
Tanjiro laid back and let her do it. Nice and slow, with a good amount of wiggling her hips, she very gradually was able to force the head in.
"Ahh!" Nezuko moaned loudly as the head slipped inside. She could feel it stretch her out in all the best ways. It was a strange sensation, but it felt nice. After a moment to get used to it, she looked down at her brother. "I'm gonna start moving now…"
Tanjiro nodded. As she began to slowly grind up and down, moving her small hips back and forth, she slowly was able to accommodate more and more of his shaft. Every downstroke slipped a little more inside.
Tanjiro held his hands towards her, something to hold onto. She graciously accepted as she was about halfway down his shaft. With a little more work, she was finally able to take it all. Once fully hilted inside his sister, Tanjiro noticed that a small bulge was visible just below her navel.
Nezuko on the other hand was just trying to keep herself from moaning out loud. Her face was bright red with blush, and a couple of beads of sweat were forming. She felt so much pain and pleasure now that she had bottomed out on her brother. His dick filled her so much, it was painful but felt so good at the same time. Good on a primal level, she could feel places inside herself she never knew existed.
"Are you ready?" Tanjiro asked.
Before Nezuko could ask what he meant, she felt his legs shift under her. He had brought his legs up, bent at the knees, and planted his feet on the bed. Still holding her hands he slightly began to push upwards. Tanjiro was now in a position to start thrusting from below.
Nezuko nodded, unsure if she could handle it.
Without any more hesitation, Tanjiro began to slowly thrust back and forth in and out of Nezuko's small form. After a few strokes, Nezuko tried to do her part, trying to synchronize herself with the thrusts, lifting herself on his downstroke, and going down when he thrust up. It was difficult and a bit clumsy, but the result was harder thrusts as their bodies connected with even more force.
Each hard slam echoed out into the room with a wet slap, accented by their heavy breathing and soft moans. Additionally, Nezuko could feel a shock wave run up her body every time their pelvises met.
As their illicit lovemaking continued, it got more feral, more primal. As Tanjiro was being brought close to the brink once again, his pumping became more frantic, more erratic, as if desperately trying to find release and fill her womb. It was something that Nezuko quite enjoyed. The feeling of being so attractive to him, that he simply couldn't wait to pump his seed inside her. It made her feel desirable, wanted, loved.
As the last few strokes were going to push him over the edge, Tanjiro fiercely grabbed Nezuko's thin hips, holding her steady for the last stretch.
"Nezuko, I'm going to…" Tanjiro said through ragged breaths.
Nezuko only nodded at him with a smile.
Tanjiro slammed hard into her small frame three more times, staying buried deep inside on the last thrust. Nezuko could feel his dick swell and twitch. A moment later, she felt a burning heat pour deep inside her.
Another moment and Tanjiro's grip softened, and he laid his head back on the bed. Nezuko could feel his spent load gestating inside her tummy. She leaned down and laid on him, her ear on his chest, hearing it beat wildly. She also felt his dick slip out of her as it rapidly deflated. She was tired, and they were both sweaty. But being close like this felt nice.
Tanjiro wrapped his arms around her, hugging her gently as she laid atop him. He was exhausted, but this kind of relaxation is exactly what he needed.
"I love you," Nezuko whispered.
"I love you too, Nezuko." Tanjiro smiled and put a hand on the back of her head, stroking her hair.
They could both feel the weariness of the day and tonight's activities dragging them down to sleep.
The last coherent thought that drifted through Tanjiro's mind was 'I wonder if she can get pregnant.'