A new sensation is felt at your butt hole, as your brother begins to pick up the thrusting pace, you feel something warm and liquid splash against the walls of your anal cavity. There seems to be something attached to his cock and begins to swell as it thumps against your rear entrance. The sensation feels amazing and then something else happens. The swelling part of his cock pops inside of you, sending him into a massive humping frenzy and shivers of ecstasy through your body.
As your brother's hips pound your sex hole like a jackhammer, that swelling on his cock gets really big inside and begins to hit a spot inside you. This causes your own dangling cock and knot to swell to full size. His heavier breathing and panting in your ear is all that is needed to send you over the edge and start your new doggy cock squirting.
His humping then gets shallow, as his swollen cock now fills your cavity. That spot his knot was hitting before now has serious pressure being applied to it. His knot is actually throbbing against it, forcing your own cock to squirt and throb even harder.
The last remnant of higher reasoning fades away, as the two of you totally succumb to the bliss of mating. You turn your head to see that your body has now completely transformed to a male German shepherd's form. You turn you head back and see the same has happened to your brother.
The smells of your mutual satisfaction and mating saturate the air of the hallway. Canine moaning sounds begin to leak from beyond your parent's bedroom door. This only adds to the sexual bliss of being tied for the first time ever. Your brother seems to show no interest in doing what most dogs do at this point. Instead he simply stays on your back and begins grooming your neck and ears. You lick his muzzle in return affectionately.
Then some barking is heard from behind you and through the door...