Ty grinned up at Dustin, licking a stray bit of cum from his lips. "Now you can fuck my pussy, Dusty."
Dustin nodded in understanding, and slowly slid off the jock strap. As he reached up to take off his bow tie, Ty raised a hand. "No, leave the tie on for now, Dusty." Dustin nodded again, and climbed into bed with Ty, straddling him. He grabbed his big dick with one hand and aimed it at Ty's new hole. Slowly Dustin pushed in, feeding half his dick into his foster brother's vagina. Ty moaned softly in approval, loving how much better this felt than just stroking his experimental genitals by hand. Dustin moved back and forth slowly, each thrust feeding in a bit more of his sixteen-inch monster. Ty let out a cry of lust, reaching up to grab Dustin's beefy shoulders for support. He had already made it so that his foster parents wouldn't come in his room or pay any attention to sounds coming from it, so he didn't hold back moaning with delight as Dustin slowly plowed him.
"Harder!" Ty grunted, reaching around Dustin's body, digging his fingernails into his hunky brother's back. Dustin responded immediately, ramming his dick in with increased force. "Yes! Yes!" Ty cried out, running his hands up and down Dustin's back. Suddenly inspired, he willed Dustin's muscles to grow larger and stronger. Ty moaned softly as he felt the body looming over him respond to his will, the muscles starting to swell, growing under Dustin's skin. The force behind Dustin's thrusts increased as well, powered by his growing strength.
"More!" Ty demanded, his mind starting to cloud with lust. Dustin's muscles exploded with size, bloating dramatically, swelling with power, the bow tie ripping off as Dustin's neck became too thick for it. Dustin's thrusts likewise became increasingly forceful as he powerfucked Ty with his giant dick, his huge muscles putting incredible strength into his fucking. Ty lay back, his mind buzzing with pleasure as Dustin jackhammered him with a sixteen-inch cock. "Now cum in me!" Ty gasped out, on the verge of passing out from too much stimulation. Dustin roared, every huge muscle flexing at once as he came, shooting a massive load of cum into Ty. Ty cried out as well, his new equipment orgasming over and over as it was pumped full of muscleman seed.
After a minute of being filled, Ty was finally feeling sated. "Okay, Dusty. You can stop now. Get up." Dustin nodded and complied, pulling his huge tool out of Ty's cum-filled pussy, standing by the side of the bed. Ty got a good look at his brother. Dustin was now 350 pounds, packed full of bloated muscle, his huge dick still hard, a pair of big balls hanging under it. Ty smirked at his handiwork. There was something exciting about making such a magnificent beast and yet having total control over it. "Put your jock back on. And put on another bow tie. My servants have to look good." Dustin nodded, pulled on the jockstrap, and then strutted out to his room to get another tie. Ty lay back on the bed, grinning as he felt his foster brother's cum slosh around in him, plotting what to do next.