Disclaimer and all that: I borrow a lot of quotes and ideas from just about everything I read and see. If you see something here that is truly your idea, and I failed to give you credit, just drop me a line so I can correct that. Besides that, the Harry Potter Universe is J.K. Rowling's. The Callahan sisters own themselves. I'm just the puppet master, and I should really get paid for this.

Kerri Potter and the Little Sister of Doom: Sixth Year

Chapter One: Pets and pests.

It was November and snowing, all of Hogwarts was freezing. Slytherin dorms are in dungeons, Kerri's toes went numb immediately as they touched the stone floors. She had her evil green fuzzle slippers charmed to warm, and pulled on the closest robe. No matter that it was the robe she had tossed onto the floors just last night. Wrinkles have never killed anyone.
After brushing her teeth, Kerri splashed water onto her face, and trotted up to breakfast still in her pajamas, wrinkled robe over top in a half-hearted attempt to follow dress code.

She passed her sister, who was doing detention with Professor Flitwick. Krista Callahan was writing on a muggle chalk board, "Hedwig will not become a phoenix, no matter how many matches I light underneath him." It looked like she had been there for a while, hundreds of sentences had scrawled over the board. She didn't look properly chastised, however. Kerri peered harder into the classroom from around the corner, and nearly jumped out of her skin when the Real Krista tapped her on the shoulder.
"Nice slippers." She was trying to stuff an entire pancake into her mouth while talking, and had a three foot tall stack floating behind her.
"Are you making your clone serve your detention?"
Krista tried to look her most innocent, the effect was ruined because she still had a whole pancake stuffed into her mouth. Her cheeks puffed out like a hamster.
Kerri wished she had a clone, but she wouldn't admit it to her sister. She just turned away in disgust and stalked to the kitchens.

Pancakes and pumpkin juice later, Kerri was at a loss of how to spend her day. She was crap at sport, and wasn't about to break her neck on a broom... Then she had a wicked idea. An awesome, scandalous, and great idea. She dashed back down to her dorm bed, and threw on her Hogwarts uniform, not even bothering to shimmy out of her pajama boxers. They were green, like most of what she owned. Kerri, if given the choice, only wore green, silver, and black. Piled at the corner of her bed was a green and silver scarf, and entwined in her house scarf was a dozing snake.

Voldemort the snake. I'm sure you remember him from the other adventures of Kerri Potter, if you don't just nod your head and pretend that you do. Kerri decided it was time to teach her snake a few tricks, inspired by Krista's use of her clone.

After a shower and a ten minute argument with the mirror about the state of her hair, ("Shave your head bald, or wear a paper bag and no one will complain!") she made her way back out of the dungeons and into the Great Hall to catch a ride on the moving stair cases. No one really made any disparaging remarks about her hair anymore. During the summer Kerri had convinced her mom to let her bleach two chunks of blonde into the front of her hair. By convince, we mean, Kerri bleached two chunks of hair to a light blonde, and showed up to dinner with the new hairstyle. Her mother hasn't killed her yet.

Also, dear reader, note that Kerri had her pet snake wrapped around her neck, under the scarf, to keep warm. The dungeons of Hogwarts were freezing in the winter, and snakes are cold blooded. She didn't particularly fancy having a snake Popsicle for a familiar.

Kerri was on her way to the Hogwarts library. Of course should have to bump into the annoying pest known as her sister on the way. Krista had her clone trailing behind her; covered in chalk dust, carrying all her books, potions supplies, and balancing a fishbowl on her head. Kerri wasn't about to ask what the fishbowl was doing there.
Krista, the original, poked at the sleeping snake. She prodded. She danced and weebled and wobbled, hissing like mad.
"Just what do you think you are doing?" Kerri demanded.
Krista's clone, was mimicking the weeble-wooble hissing dance. Water from the fish bowl sloshed onto the mini-Krista's Gryffindor scarf.
"I'm talking to your snake. Duh."
Voldemort peeped one eye open and hissed to Kerri. "I'm sssleeping. Make it go away."
"See! It's talking to me!"
"Voldemort says he is sleeping. Go away."
"You named your snake Voldemort? How cliché."
"This coming from the girl who named her clone Mini-Me."
"At least mini me is fitting. It accurately describes my clone; a miniature version of me. Voldemort is French for flight from death. Where the crap are your snake's wings?"
"I didn't name it Voldemort. He is Voldemort!" The snake in question bobbed its head as if in agreement before settling into the Slytherin scarf around Kerri's neck.
"Sure. Whatever you say." Turning away from Kerri, Krista motioned to her dancing clone and twirled a finger by her ear. "She's gone cuckoo. Lets go."
The Krista-clone stopped dancing, but started yelling, "Cuckoo! Cuckoo!" Like the clock, and they walked away.
No one really believed her, but her pet snake really was the once evil, murderous You-Know-Who. Everyone just went on believing that the lazy, good for nothing, Boy-who-lived-to-be-a-pain-in-the-ass, vanquished him; just like the prophesy said.
Only Voldemort, Kerri, and Harry Potter knew the truth.
Kerri Potter was now spending her day in the library showing her pet snake a revised list of rules on being an Evil Overlord. She figured she could teach the snake a thing or too about the subject, seeing as she wrote the list herself. Her wicked plan was set into motion, she was going to make a bigger, better, more efficient evil pet. If the plan failed, she could always teach the snake to fetch, or something.
On the cold November morning, Kerri sat, quill and parchment in hand, talking to the snake. Even Madam Pince stopped drifting over to the corner. Apparently Krista wasn't the only person to consider Kerri cuckoo.

"Honestly, Volde. That black robe and scary mask thing is way too Halloween. You should wear bright colors. It'll confuse your enemies. If not confuse.. then just wear neon. You could try and blind them."
The snake just hissed his disapproval.
"Oh, here's a good one. Number 38: If an enemy I have just killed has a younger sibling or offspring anywhere, I will find them and have them killed immediately, instead of waiting for them to grow up harboring feelings of vengeance towards me in my old age."
Now I'm not sure if you have ever hear a snake growl. I doubt they have the ability. Volde did, however, make a good show of it. He also called Kerri some names that aren't acceptable to type here, if I want to keep my G rating.
"Yeah, you're right. Maybe you should have read this list before you had that brilliant idea. Hey! Hey! Those aren't nice words, don't make me wash your mouth out with soap. It's not my fault your avada kedavra sucks. "
The snake mumbled something that sounded suspiciously like Avada Kedvra towards his owner.
"Too bad you can't use magic anymore. You might give me another scar. Everyone is wearing them these days- the height of fashion and all that."
Voldemort refused to speak to her, or listen to her from there on out. He snuggled once more into the soft fleece of her scarf, which was his own snake version of singing LALALA and plugging his fingers in his ears.

Lets stop the story in the library right now just to clarify some things.
Kerri's surname is not, in fact, Potter. Her fellow Slytherins gave her the moniker in her first year at Hogwarts. Mostly they were teasing the her for having a scar on her head. It was meant to be a joke, to insult Harry just as much as Kerri. It sort of stuck.

Kerri used to follow Harry Potter around during their first year at Hogwarts. Ms Rowling failed to mention that little bit. Honestly, she had some sort of obsession with Harry really, and neglected over half the on-goings at Hogwarts. I'm getting a head of myself though...
See Kerri did have a lightning bolt shaped scar on her forehead. When she was much younger, and not at Hogwarts, Kerri was learning to ride a bike down a hill. She fell comically all the way down only to have her tumbling-fall broken by a large jagged rock.
Which only left a sort of jagged acute-angle-shaped scar. Her little sister, Krista, poked at the scar with her wand, (while Kerri was unconscious from a rather vicious game of Knock Kerri Unconscious) and charmed the scar a more noticeable lightning shape.

(It is a little known fact that Krista has a scar charmed in the shape of an Oompa Loompa on her bum. Sometimes it sings and dances at really quiet formal meals, and Transfiguration tests.)

Around now is where we are going to learn about Kerri's older sister, Caitlin. Caitlin Callahan was a year older than Kerri and in her seventh year at Hogwarts. Caitlin was a lot more experienced at getting herself in and out of trouble.
Speaking of trouble, that happened to be Caitlin's middle name. Literally. Her name was Caitlin Trouble Callahan. She was a Prefect, a Chaser on the Quidditch team, and an all around good girl. At least that's what her professors thought.
Caitlin had a habit of sneaking out of the common room for a late night snack from the kitchens. She also met with someone in Moaning Myrtle's bathroom regularly. By regularly we mean at least once a week.
Caitlin's two younger sisters made her look bad. They got into a lot more trouble, and were caught for it. They lost a lot of house points, and when they weren't fighting with each other they were causing trouble within their houses. They were the subject of many Prefect Meetings. Everyone pleaded with Caitlin to control her sisters, or talk some sense into them, or to beat the living snot out of them, or something. The Slytherin's said they would take care of Kerri... who in the most part behaved. She was just a little... weird. Krista was out and out loony, and could be heard laughing loudly to herself on many occasions... and she had this odd habit of being in two places at once.

Caitlin largely ignored her two little sisters, and tried her best not to be associated with them at all. As far as she was concerned, they weren't her problem just so long as she didn't catch them out after hours. They were now the problems of their Heads of Houses, and the Headmaster. Caitlin washed her hands of the two annoying trouble makers.
Caitlin had bigger plans than being Prefect. Much bigger plans indeed.