I believe this to be overdue...Sorry...

All around them were shops out of a fairy tale of the medieval ages with ware to match. There were cauldrons, toad and newt parts, various floating, twirling, or color changing objects, feather quills and ink pots, and even sleek and shiny broomsticks that seemed utterly useless for cleaning.
Katherine moved slightly behind Lydia to keep herself from scrutiny and Amelia hustled them along as they paused a little too long by a defense shop. "First things first," Amelia pursed her lips, "We need to get you a cauldron."
"A cauldron, you mean, for like witch's brew?" Lydia scoffed. Katherine didn't say anything, but her expression agreed with her sister.
"No, for various magical potions that can do anything from dying hair, to regrowing bones, to even putting someone into an eternal sleep other than death," Amelia lectured sternly, "Never underestimate a potion. They can be poison or the antidote, and without proper knowledge of which is which, children born from a squib like you, veritable muggleborns, will be dead in a few moments."
Lydia's eyes widened and Katherine grabbed her arm tightly.
They walked through the arch and one wild and less-than-pleasant cart ride later Lydia and Katherine both had a bag full of gold to get them through the school year.
"I'll take you to buy your pets later, after my meeting," Amelia said looking at an odd pocket watch, "I'm already a bit late. I'll show you to Madam Malkin's and you two can get everywhere from there right?" The two nodded their agreement as they walked out of Gringotts. Madam Malkin's was right across the street from Gringotts, and therefore was quite easy to find. Amelia gave them each a quick hug and then set off in the general direction of the Leaky Cauldron.
"Well," Katherine looked at Lydia half-heartedly, "I guess we should go in?" The girls each took a deep breath and walked into the shop.
They were greeted by a short, overly cheery, and rather plump witch, who immediately started to ask them questions. After a rather long interrogation and an even longer fitting, the girls walked out somewhat overwhelmed but with their new school robes. They got through all the other stores without incident and finally were on their way to Ollivander's, to get their wands. As they walked into the slightly dusty shop they were greeted by a tired and quiet voice. Both girls jumped, the shop appeared empty.
"Hello," said the person, who had just appeared from a back room, "Which one of you would like to go first?" Katherine hesitantly raised her hand.
"I'll go first." It only took a couple of wands before she found one that suited her, yew wood and dragon heartstring. Smiling she paid for her wand and bounced back toward where Lydia was nervously waiting on a spindly little chair by the door.
"Your turn!" Katherine said. Lydia nervously stepped up and the little tape measure began to measure her. After a little bit, Ollivander began handing her wand after wand, after fifteen more minutes of waving random wands Ollivander handed her one and immediately pink sparks flew out of the end.
"Much better," said Ollivander, "It suits you perfectly, it's elder and phoenix feather." Lydia paid for her wand and walked quickly back to where Katherine was waiting for her.
"Let's go," whispered Lydia, "this place creeps me out." The two girls walked quickly back out into the sunshine.
"Lydia," said Katherine, "what was the name of the place that Amelia said to meet her?"
"I don't remember, but I know where it was! Come on, I want some ice cream!"
Lydia grabbed her sisters hand and the two girls ran over to the Fortescue's ice cream shop where Amelia had told them to meet her. About fifteen minutes later as the girls were enjoying their usual flavors of ice cream (it had taken a while, but eventually Lydia had found her vanilla ice cream and Katherine had found her chocolate among the multitudes of other random ice cream flavors.) Amelia came and joined the girls at their table.
"Well," she started gleefully, " how much about Hogwarts have you two figured out yet?"
"Not very much," admitted Katherine.
"How about the houses?"
"Oh! I remember those," exclaimed Lydia, "Or at least some of them, I remember someone saying something about Ravenclaw and Slytherin, but I don't remember the other two."
Amelia laughed, "The other two are Gryffindor and Hufflepuff. I was in Ravenclaw myself, so I'm a rather biased source regarding which one you two want to be in. I had an excellent time in Ravenclaw. But if you ask any wizard or witch from a different house they'll tell you that their house is the best. Hopefully you two end up in the same house, that way you'll at least know each other. Even though I was a Ravenclaw, our family was typically sorted into Slytherin in the past."
"What do you mean our family?" Katherine queried curiously.
"Your mother really didn't explain any of this to you?" asked Anna in astonishment.
"Nope," Lydia popped the 'p' at the end, "Should she have?"
"It doesn't matter, anyways you two come from a very long line of pure bloods. You're from the Gray family, your father is a wizard who had his wand snapped for reasons I can't go into. Your mother is a squib, so she's a pure blood who was born without magic. That's why she couldn't go to Hogwarts."
"Ohhhhh," the twins nodded exaggeratedly in unison.
"Now we'd better get going," smiled Amelia, "you two have a lot to learn about the wizarding world still." Her grin widened, "I spoke with your mother and she said that you two can stay with me for the rest of the summer if you want to, I'll give you some time to think about it, but before we go I need to get you two your birthday presents, I believe I promised you both a pet?"
The girls grinned excitedly.
The three headed off to the pet store across from Flourish and Blotts. As they walked in Lydia headed straight toward the owls and Katherine wandered over to the kittens. Twenty minutes of petting later Lydia walked out with an owl that was black with dark brown eyes that almost matched her hair color, and Katherine had a cute little black/gray kitten with bright green eyes.
"So," said Amelia, " would you like to stay with me for the rest of the summer?"
"Will we get to see our parents and Annie before we leave for school?" asked Katherine anxiously.
"Of course!" replied Amelia, "and you can use Lydia's owl to send home letters everyday if you want."
The girls had a brief muttered discussion amongst themselves, "We'll stay with you" they chorused.
…
The next day Found them in Flourish and Blotts once more. After much begging, and pleading, and completely shameless crocodile tears, they convinced Amelia to bring them back for some "extracurricular" books. As they dashed through the store, never looking at a book the other had already examined (these twins are weird that way), Lydia tripped over an ill placed and ill-intentioned foot into Katherine, who unfortunately couldn't support her weight an stumbled into the a certain George Weasley's arms. Also unfortunately for him the combined weight of the two girls sent him tumbling backwards into a rather unstable bookshelf. During the resulting chaos there was a mutual agreement amongst the four unintentional culprits to run. After a few minutes the four sat down heavily on the steps outside a random shop.
Panting, Lydia glared at the two redheaded twin boys sitting by her and her sister, "Now look what you got us into?"
"Lydia, it's not this one's fault, and I'm sure his twin didn't mean all that to happen..." Katharine reprimanded timidly, still holding her sister's arm.
"HE TRIPPED ME!" Lydia said angrily, pointing dramatically at the guilty party, "People don't do that!"
"Well," The redheaded twin she accused rubbed the back of his neck with a slight scowl, "It's not like I thought you'd be so heavy George couldn't even hold you up."
"ARE YOU CALLING US FAT?"
"WHAT THE HELL IS WRONG WITH YOU!"
"GIT!"
"PRISSY!"
Katharine then made the mistake of letting go of Lydia's arm.
The two ran at each other like angry cats, scratching and scuffling and rolling around on the floor.
"I'm Katharine," Katharine held out her hand to George tiredly as Lydia chomped rather viciously on the other twin's ankle.
"George, nice to meet you," George sealed the greeting with a handshake just as Fred managed to shove Lydia a full four feet away.
Lydia threw a well placed spin kick at his knee. He fell down to the ground with an omph.
"Oh wow," Katharine breathed at the awesome display of martial arts.
"Er, I'm going to collect Fred now..." George pointed at his downed brother apologetically and Katharine nodded understandingly.
"I should probably check Lydia's injuries, too..." Katharine fretted, noting the multitude of scratches marring Lydia's skin.