Todd heard Ann moving around in the bathroom. Pressing his goat ear to the door, he could hear the smallest of sounds. The clink of the towel bar as his sister slid her towel off, followed by the swish as she wrapped it around herself. Slowly he opened the door to the bathroom.
Peering inside he saw his reflection in the mirror. His furry ear, one horn, and his right side of a goat head peered at the sexual quest. He paused a moment to marvel at his strange, square pupil. The human in Todd knew that he had become something not seen in this world for thousands of years. But the goat only cared about the naked, fertile young woman just a few feet away.
Ann had one foot on the rim of the tub, drying her feet and bent half over showing her rump to Todd. A smooth expanse of white sister-flesh, above the taut muscles of her thighs. He couldn't see her face, or the generous breasts he had often fantasized about. But he didn't need to. In a second Todd moved in and grabbing his sister, trapping her in the revealing position.
Ann screamed first out of disgust and continued out of the fear of hairy arms and black pointed talons. The position was just perfect as Todd's goat cock, oiled with cum, thrust deep into Ann's pussy.
This wild animal act of incest started, came and finished so fast that Todd longed to mount her again and again. He still held his sister, pressed into him, standing in a spooning position. His human arms were wrapped around her waist, and one taloned hand caressed her breast. His goat cock lay along the line of her firm human ass.
Ann gasped for breath, too shocked to process what had happened to her. Raped from behind, the actual sex had taken less than a minute. But she remained locked, helpless, in these... arms, these strange inhuman arms that held her. And the hand on her breast, the breast still heaving from the struggle of the rape, that hand touched her almost lovingly. Like the touch of an old friend. The mixture of human tenderness and inhuman force left her...
...left her, she realized, wanting more.
Ann leaned back into whoever had just raped her, deliberately pressing her ass into his groin. She stroked her hands along his strangely hairy arms. "Is that all you have in you?" she purred. "Or are you afraid of a real woman?"
Todd's goat cock twitched at his sister's seductive voice. Though he'd fantasized about her ever since he realized what made women desirable, he never believed he'd ever have her naked in his arms. He'd already fucked her, releasing possibly gallons of goat sperm into her fertile young body, so the animal was satisfied. But the sound of her voice, the warmth of her skin, fired the human in him. He spun his sister around and pressed her against the wall with his entire body.
Ann's eyes went wide when she saw the thing now facing her. Not human, but not a dumb animal either, it was... gorgeous! It had goat pupils inside human eyes, a fine line of goat fur over most of its body, thick buck teeth perfect for chewing through branches and roots, and a strange little half-smile on its lips. A half-smile she'd seen before. She put a hand on its furry cheek and stroked it as she tried to remember...
"Todd?"
The realization that this thing was her younger brother washed over Ann with a wave of dread. Under some kind of, who knows, a curse or something, her own brother had raped her, quick and hard, with the ferocity of an animal. It wasn't violence she'd felt as her brother had forced himself up into her, she realized. It was pure need.
Todd looked at the turmoil of emotions on his sister's face: horror at the rape, followed by disgust at the incest—then, suddenly, tenderness. Below his waist, the goat was getting hungry again, his hairy shaft back to fully erect and just inches from his sister's opening. But his human brain wondered. Why, just seconds after he had raped her, did Ann look like she was falling in love?
Unsure what to do next, Todd decided to obey the goat. He leaned forward and licked Ann's cheek, running his thick, rough goat tongue over his sister's smooth white face. Brother and sister both trembled, knowing what was coming, scared of the high feelings, but both wanting it. Ann grasped Todd's face and guided it to hers. They kissed.
That was all the slowness the goat could stand. Todd lifted his sister off the wall and pushed her roughly onto the floor, grasping her breasts in both his hands and thrusting roughly into her.
"Oh!" Ann cried at the ferocity of her brother's entry, and arched her back. As he pounded into her, she pushed off the floor to meet his thrusts. "Fuck me, Todd," Ann growled, holding his hips as he held her breasts. "Fuck me. Fuck your big sister."
Sister and brother quickly found the perfect rhythm, answering an untapped primal need in both of them. Soon even Ann's words stopped, replaced by a single, bleating cry. Sister and brother, human and goat, came together like a perfectly engineered machine.
So enraptured were both of them by their primal sex, they failed to notice, amid their paired building orgasms, that Ann's body had begun to change. Fine, dark goat hair began sprouting all over her face and body. The nubs of horns began pushing at her brow. And her legs, curved back from the force of their incest, realigned themselves into goat legs.
As Ann's pussy clamped down on her little brother's dick, and Todd released gallons of semen into his big sister's unprotected womb, the final transformation overcame her. Ann was now a complete goat girl—and her brother's perfect mate.