It was true. As he straightened, his huge bulge made its presence known, leaving no room for doubt.
Yumi's smile widened, contented and mischievous. "Well, you helped me, so it's my turn again." She slowly reached out to release his cock from confinement. "I can't just leave you like this."
Her touch sent shivers down Seiji's spine. It was as if he didn't cum earlier at all. Her fingers circled the base, thumb rubbing the head with a skilled precision. "It's so…" She glanced up at Seiji, without finishing her sentence. "Never mind." She said, stroking him slowly.
Seiji's hips jerked involuntarily as she did.
"Your body reacts so honestly," she whispered. "I like it."
Yumi's eyes darkened, and she raised her shirt, exposing one of her breasts. She was now sitting up, and Seiji's cock was right at the breast level. She moistened two of her fingers in her mouth and spread saliva onto her nipple.
"You made me feel so good," she whispered loudly. "Let me return the favor."
Seiji whimpered as she began rubbing Seiji's cock against her nipple in tight, slow circles, the sensation he'd never experienced in his life.
"Oh, sis!" he moaned loudly, making her smile wider.
"You like this?"
"Yes," he groaned.
She flicked her nipple repeatedly with his cock, gasping with each contact. "Did you say a bad word?" she moaned.
"No…" He was a well-behaved boy.
She circled the nipple again, a little faster. "Well, you should. You're having sex, so you can say bad words."
Seiji moaned, his knees twitching. "Fuck…" he mumbled.
"I can't hear you, little brother?" she cooed, a mix of mockery and seduction.
"Fuck!" Seiji groaned louder. "I like it so much… "
She watched his suffering in fascination, dragging her nipple slowly along his entire length. "It's so long," she whispered to herself.
She increased the speed again, and Seiji's while body tensed like a wire pulled to snapping.
"Too much… I'm going to cum soon…"
"That's okay, Seiji," his sister purred. "Then cum. You can cum on my boobs."
Seiji's eyes were fixed on Yumi's breasts, as she began to stroke him faster. Soon, he felt the familiar tingle at the base of his spine, his balls tightening. But this was a million times better than wanking, so his eyes simply rolled back in his head.
"Cum," Yumi urged him, and grabbing onto her shoulders for balance, Seiji with a groan started shooting ropy strands of his cum right onto Yumi's breasts. Yumi's eyes gleamed with satisfaction as she circled Seiji's cock around her swollen nipple, white liquid dripping all over her.
When Seiji's cock finally stopped contracting, he opened his eyes again. "Holy crap," he whispered. "Thanks, sis… That was AWESOME."
"It was," Yumi murmured, her fingers slowly rubbing his cum into her skin, watching it get watery and slide down her body. "First you helped me with my milk, and then I helped you with yours." She raised her eyes, looking impressed. "You also had too much." She traced the beads running down her belly. "Look, it's all over my body."
Seiji felt tingling in the back of his head from pride. He couldn't take his eyes off her cum-covered skin. She was right: it was his cum, and it was all over his sister's body!
"Thank you, Grandpa," he thought, almost like a prayer. "This is the happiest day in my life!"
Yumi straightened her panties and got up, looking around the room.
"I'll take a shower before dad comes home," she said, winking at Seiji.
"Ah," Seiji replied, as if waking up. "I'll go wash up, too." He turned and rushed back to his room, his mind still flooded.
He touched the amulet. Grandpa wasn't joking. This thing was already changing his life!
Seiji pictured cum-covered Yumi and put his hands together, bowing his head.
"I'll do what's right, Grandpa. I'll honor you and thank you for what you've done for me! I'll go to the temple and pray and set up incense in your memory." Seiji closed his eyes. "I should've been nicer to you when I was a kid. I was so stupid, please forgive me, Grandpa!"
And Seiji clutched his amulet, before turning on the water.