You stare at your brother, now half dog. "We need to stop," you whisper, your voice trembling from the powerful orgasm he already gave you, "before we end up as dogs."
Your younger brother remains kneeling in front of you, naked, his ten-inch cock still in his hands, your cum still covering his muzzle. His eyes still look mostly human, but with his German shepherd ears and muzzle already in place, he already has that sad, puppyish look to his face. He looks up at you, quivering with arousal, as you stand equally naked before him. "I don't know if I can, Sis," he whispers. "I don't know if I'm strong enough. You need to stop for both of us."
You drop onto your knees in front of your brother. You grab both his hands in yours, steering the one away from his already throbbing cock. "You have to think," you tell him. "Think like a human. I'm your own sister, your older sister. Remember? Not just some girl. We used to fight over the armrest in the movie theatre when we were small. You called me Pizza-Face. I'm your sister. Remember?"
Suddenly your brother clenches both your hands in his. "I remember, yes. But I feel something now... I don't know, I've never felt this before. Am I in love with you?"
"No, no. It's the doggy pheromones, they're clouding your feelings. You have to think, not just feel. I'm your sister. Think about that. Think with your brain."
Your brother squinches his eyes shut and curls his doggy lips up, looking like a dog-boy struggling to lift a massive weight. "I can only think of the feel of your hands in mine right now. I can only think of how sweet you tasted as I ate you out. I know you're my own sister, but I feel like you're the only woman for me."
You look at your brother's straining face. You have to admit, he's cute, halfway between human and dog. You feel your heart start pounding in your chest. The feel of his hot doggy breath on your face makes your nipples stand up—all six of them. You thought having two visible nipples through your t-shirt was embarrassing? What about six naked nipples, standing erect and pointing at your own brother?
Finally his head drops and his eyes open. He pauses a moment, then looks right at you. "I don't know what we are," he says. "We're something brand new. Thinking is... is weak, somehow. I just know I want you. I love you, sister. Only you."
Slowly, your brother drops your hands. He slips one arm around your waist, pulling your body into his, and puts his other hand on the back of your head, guiding your faces together for an incestuous kiss. It takes you three tries to work around your muzzles, but finally, your lips meet, and your long, flat, doggy tongues entangle.
The feelings are just too strong. Your brother's body is responding to your pheromones, and your body is responding to his. Your brother's naked chest presses against your six breasts, and you never knew you could feel something so electric. You're both in thrall to your canine passions.
He isn't your brother anymore. And you aren't his sister. You are a woman and a man, who know nothing but the urge to bring your bodies together.
Your brother lays you back across the carpet, slowly and lovingly, a move that's surprisingly human, and all the more so when he straddles your body and thrusts his penis into you with animal force. You no longer want him to stop. You only want to feel his full ten inches inside you, and as he kisses you gently, as he handles your breasts roughly, you get your wish. Your brother thrusts, fierce, hard, down on you, and growls like a dog as his paw-hands explore your body. You love this. Oh, oh, brother. Oh.
"Cum inside me," you say out loud.
"Yes," he replies, with a growl.
"Oh!" you gasp. "I'm not a virgin anymore."
"Me neither."
Then as one, you and your brother arch your backs and howl like wild wolves. Your vagina clenches him, again, again, as he empties himself into your naked womb. Oh God, you think. I never want to love anyone but my doggy brother again.
At last he collapses on you. You hug him and stroke his furry back while you both struggle for breath. "I love you, Sis," he whispers.
You look down at your bodies, still tangled together, and are astonished to see...