Suddenly you got the wild idea to give your sister cow udders, and just as the idea popped into your head you immediately saw it begin to take effect on Jessica's body. The space between her belly button and crotch began to bulge out and turn bright pink, and 4 long thick nipples started to grow out of it. As it became more defined the huge milk sack hung down off her tummy like a basketball, sloshing with a fresh bounty of delicious milk. Her already huge breasts expanded even further as well, those engorged mammaries also leaking a reinvigorated supply of creamy sustenance.
But that's not all that happened. You looked up at her face to see that she now had a pair of large, black and white spotted cow ears sticking out from under her blond hair, as well as large thick horns! Similar black and white hair also began to develop on her legs as well, and her feet had turned into hooves! Her thighs begin to thicken up and her hips widened, and in moments her entire lower body had become that of a cow, with thick powerful haunches and a tail to boot.
Just as you resumed suckling Jessica's leaking teats you both looked up to see the door to her room open! Standing in the doorway was your mother, inexplicably carrying a stack of large buckets with her.
"Oh, I guess you've already started," she said, stepping into the room and setting the buckets down. "Hope you didn't drink it all this time."
"What are we...?" You started to ask as Jessica got up to meet your mother.
"It's milking time again, silly," your sister giggled. "Don't worry mom, he only drank my boob milk. My udders are still super full. We were just warming up."
"I can tell," your mom commented, kneeling by your sister and caressing the swollen milk bag, making her moan slightly. In a routine they seemed to have rehearsed countless times your sister went over to her bedroom wall and put her hands against it, legs back and spread as if she was about to be searched. Your mother came up next to her and placed a bucket underneath her dangling udders, then looked back at you. "Wanna help sweetie?"
"Sure, what do you need?" You asked.
"Well, you know happy cows give the best milk. And I know you know how to make your sister really, really happy." Your mom gave you a dirty wink, and you immediately knew what she was insinuating. You didn't even question that this was suddenly happening, it was just too good of an opportunity to pass up.
"Yeah Ben, come make me feel good," your little sister said seductively, wiggling her black and white spotted booty and swishing her tail as you.
You got up and stood behind Jessica's large cow butt, your dick already at full mast again as you marveled at her bovine pussy and ass. Her feminine yet animalistic musk was extremely powerful, triggering something within your primal instincts telling you that you needed to breed with her right away. You pressed your member up to her thick, moist pussy lips and moaned as the powerful warmth immediately overwhelmed you, her powerful pussy immediately gripping your shaft. As you started thrusting your mom started milking, sending Jessica into a frenzy of pleasure.
"Oooh, good girl, Jess! Good girl! Look at all this milk," your mother cooed as she squeezed her daughter's sensitive udders. She leaned in and took one in her mouth, sucking some sweet milk out for herself. "Mmm, delicious..."
As you started thrusting harder and faster you could hear the milk pouring into the bucket at a higher rate as well. Jessica's moaning was almost beginning to sound like mooing at this point too. Her tits were absolutely gushing milk while being pressed against the wall, though all of that was going to waste unfortunately. Not that it mattered though; your mom was already getting ready to swap to the next bucket, the first one almost completely full.
"That's it Ben, keep it up!" Your mom encouraged you, giving you a slap on the ass. "Fuck all that sweet milk out of your sister!"
"Mmm... Mom I'm... Moooooo!!" Your sister moaned loudly as your thrusts finally brought her to orgasm. Her pussy squeezed you even tighter than before, gushing her juices all over your dick and balls as her fat donut of an asshole pulsated in time. You also heard her udders begin to absolutely blast milk into the bucket, as if she were cumming through her them as well.
"Ooohh, there it is!" Your mom said delightedly as she coaxed out more of her daughter's milk-gasm, rapidly having to switch buckets as gallons of milk sprayed out. You were getting pretty close to another orgasm yourself, but before that...