As Cal turned the corner into the small room, he saw that it was completely empty. Before he could turn around, he felt himself being tackled from behind, soft breasts pressed against his back, the stench of sex in the air. It was Katy.
Cal struggled to get back on his feet, but it was too late. He felt the leather around his throat, and heard a click as Katy secured it to him. He was suddenly filled with the desire to mount his naked sister.
Katy fell on to her back, and as she laid on the floor Cal grabbed both her ankles in one strong hand (they were surprisingly thin), pushed them up over her head, and lowered himself on top of his sister. His throbbing cock found her dripping pussy and slid in effortlessly. He began to pump at her with all of his strength.
“That’s it, Cal, fuck me!” she cried. “Fuck your sister like a bitch!” He drove his manhood deeper into her.
Soon he felt the transformation begin again. This time, he didn’t—he couldn’t—stop it. He felt his cock changing shape, the tip ballooning in his sister’s vagina. She grinned an evil grin as she stroked her hands through the fur growing up his back, caressing his muscular shoulders as they became strong canine haunches. His face pushed out into the shape of a German Shepherd’s, and Katy giggled demonically as she licked her bother’s wet snout.
She cackled, “Good Cal, good doggie,” as her brother—now entirely a beast—continued to plow his bright red cock into her slit.
He was a dog now, he realized, looking down at his sister’s breasts, which bounced merrily as he pounded his doggie penis into her. He bgan to lap vigorously at her nipples.
“Ooh, that’s good!” Katy exclaimed. You’re a much better lover as a dog than as a guy, little bro.”
Cal howled and pumped harder as he shot his canine jism into his sister’s human pussy. He had to admit he liked fucking her while he was in this form, but he knew he’d have to escape, and soon, if he didn’t want to be under her power forever.