This is about Petunia and Lily as kids, and about Petunia's jealousy of Lily's magic. I've changed the canon a bit: I moved Lily and Petunia's argument from the train station to Hogwarts and changed some of the words, but that argument is taken almost word for word from the 7th book. I also added a forgetfulness spell around Hogwarts – the part about Muggles seeing Hogwarts as ruins is taken from the books.
Without further ado, here it is! Enjoy:
Jealousy.
The word was squatting in the center of the room, larger than life, permeating through the carpet and the air to the two twin beds on opposite walls. The two girls, occupants of the beds, both felt it; although it was stronger in one than the other.
In the older of the two sisters the word had been growing for a long time as it slowly consumed her. She had watched as the other one did amazing tricks she could never dream of; she had listened as that insane Snape boy filled her sister's mind with insane ideas of witchcraft; she had watched from the top of the stairs as a woman in flowing green robes handed her sister an official-looking envelope with a red wax seal. But she still could not quite believe it until, earlier that day, she had stood in the afternoon sunshine on a cobbled street in Diagon Alley and watched as her little sister waved her newly bought wand and golden sparks poured from the tip. That was the moment it had become real. Her little sister was special; different, and she was just a gigantic blob of normal. She could never admit it with her proud airs as the elder sister, but she was jealous.
The other, the young girl with dark red hair, lay awake. Her jealousy was overpowered by excitement. She was excited to be a part of an unknown world of magic, and excited to head off to such a strange and wonderful school to develop her fledgling skills. She couldn't wait to be able to come home and show her parents and her sister all she had learned. But there was a spark of jealousy in her as well, jealousy born in fear of the unknown that became stronger at night. Apart from what the kind lady in green had told her and what she had seen today shopping, she knew nothing about this school to which she was headed. She would miss her family deeply; she was only eleven after all. And although she wouldn't give up her opportunity for the world, she couldn't help but feel a twinge of jealousy of Petunia, who got to stay home with their parents.
There was a hiss in the night; a whisper.
"Lily!" It was Petunia. Lily hadn't realized she was awake.
"Yes, Tuney?" she asked.
Petunia hesitated, then began "This school you're going to…Hogwash, is it?...It sounds absolutely ridiculous, you know, but…" She sighed, and all of her haughtiness disappeared in her request, "But I would like to go with you, just to see what it's like." She finished, wistfully.
Lily had braced for an insult at the beginning of Petunia's words, but she softened at the end. She did wish her sister could come with her. She would be less lonely, and it would be nice to have a familiar face at school. She even felt a little sorry for Petunia, because all these amazing things were happening for her, Lily, and her sister was being left behind. She doubted Dumbledore would let Petunia in after seeing his polite refusal letter. But, she thought, maybe in person we could convince him.
"Would you help me sneak on the train?" asked Petunia.
Lily smiled at her sister through the dark. "Of course," she beamed.
Two days later they stood on Platform 9 3/4, Petunia swallowing nervously and Lily hugging her parents good-bye. "Look, mum! That man over there is doing magic!" Lily cried, pointing. Both of their parents turned in the direction she was pointing, searching for the man.
Petunia took a deep breath and hurriedly stepped onto the train. As was according to plan, her parents didn't notice her. She found an empty compartment and peered out the window at the platform. Her parents were glancing around worriedly, wondering where she had gone.
"Lily, did you see where Petunia went?" asked their father, frowning.
"She said she was thirsty, so she went off to find a drinking fountain." Lily fibbed.
The whistle blew, and steam poured out onto the platform from the scarlet engine. "But she has to say good-bye to you, and the train is about to leave!" their father exclaimed.
"Don't worry, we've already said good-bye, Dad." Lily smiled, hugging her parents one last time. "Good-bye, mum! Good-bye, dad!"
She pushed her heavy trunk onto the train as it pulled off the platform, watching her waving parents turn into a blur as the speed picked up. She heaved her trunk along the aisle and found Petunia just as Snape was rounding on her.
"I saw you sneak onto the train. You shouldn't be here!" said Snape angrily.
"And what were you doing watching me? I suppose you were spying on Lily again, weren't you?" Petunia retorted.
"What – no – I – That doesn't matter anyway! What're you doing here? You're a Muggle!" Snape cried.
"What am I doing here? What are you doing here is more like it. I'd much prefer a compartment with more pleasant company, thank you very much." Petunia sniffed.
At that moment Snape noticed Lily, and immediately began helping her with her trunk. "You know she can't come to school with us." He grunted as they lifted the trunk into the overhead storage. His face was still flushed with anger.
"I dunno, Sev, she really wants to come, and perhaps it'll just take a bit of convincing with Dumbledore." Lily said.
Snape sighed. Only a Muggle-born would have such a naïve plan. However, he didn't want to upset Lily be being the one to break it to her that Petunia couldn't come. "Alright, I won't stop you. Besides, we've left the station already. You couldn't get off until Hogwarts even if we did tell somebody."
Petunia settled down by the window as that awful Snape boy regaled Lily with tales of the different Hogwarts houses, and before long a boy with messy dark hair and glasses entered the compartment with a few of his friends, and got into an argument with Lily and the Snape boy about school houses. She glared disapprovingly at the boy as she, Snape, and Lily left in a huff to find a nicer compartment. But then, panic set in as she sat down again.
"Lily!" she cried, wide-eyed. "Lily, I've forgotten, I have to go feed Mrs. Ponds's dog!"
Lily frowned at her. "Mrs. Ponds moved away two years ago, Tuney, and she didn't have a dog."
"Yes, yes she does, and I need to go feed him! Oh, my goodness, I was supposed to do it at noon. I suspect he'll be starving by now!"
"Tuney, you don't have to feed any dog! Everything's alright! Calm down!" Lily pleaded.
"We're nearing Hogwarts." Snape said. "This must be some forgetfulness spell to keep Muggles away."
"Maybe we should tell someone you're here. So you can get away from this spell." Suggested Lily, who was concerned.
"No, no!" Petunia was adamant. "I want to stay! I want to go to Hogwarts! Oh no, I'm missing my doctor's appointment!"
After a few more minutes of this, Petunia began to calm down.
"The spell must only have a certain range." Snape supposed. "But I expect there'll be more spells."
A little while later, after they had changed into their school robes (Petunia putting on an extra set of Lily's, which were a bit short on her), the train stopped. They stepped out into the night and followed the group of first-years into a fleet of little boats.
"Wow," Lily gasped as they sailed across the lake and came into view of the castle. Even Snape's mouth was hanging open. Petunia, however, was agast.
"But that's nothing but a pile of old ruins!" she cried. "There's a great big sign reading "Danger!" That can't be the school!"
"No, Tuney, it's lovely! Beautiful turrets and towers and everything!" said Lily, still staring.
They reached the opposite shore and were directed to an antechamber outside what Snape said must be the Great Hall. Petunia didn't understand what was going on, as it appeared to her they were all exploring a very dangerous old ruin. A woman in green approached them, and Lily recognized it to be the same professor who had visited her to give her her letter.
"Professor McGonagall!" she cried, running up to her and clutching Petunia's hand as the students behind them continued to mill around. "Professor! I've brought my sister Petunia! I know Professor Dumbledore said she couldn't come, but I thought we'd bring her here ourselves so he could see how eager to learn she is!"
Professor McGonagall gave Petunia a sad smile. She remembered the girl from her visit. It was always hard when a Muggle sibling wanted to accompany their magical brother or sister to Hogwarts.
"I'm very sorry, Petunia, but I'm afraid it would not be possible for you to attend Hogwarts." Professor McGonagall began. Looking at her watch, she realized she didn't have time for this. The sorting ceremony was to begin in a few minutes. "Petunia, will you please wait here for a few minutes. Someone will be out to talk with you soon."
The girls both looked crestfallen, and Petunia became upset as Professor McGonagall spoke to the first years about the sorting ceremony. Lily tried to reassure her sister.
"… I'm sorry, Tuney, I'm sorry! Listen –maybe later– no, listen, Tuney! Maybe tomorrow, I'll be able to go to Professor Dumbledore and persuade him to change his mind!"
"I don't – want – to – go! You think I want to go to some stupid castle and learn to be a – a… – you think I want to be a – a freak?" Petunia whispered angrily.
"I'm not a freak." whispered Lily disdainfully. "That's a horrible thing to say."
"That's what this is. A special school for freaks. You and that Snape boy… weirdos, that's what you two are. It's good you're being separated from normal people. It's for our safety. I don't want to stay in this awful ruin with a bunch of freaks!" Petunia spat.
Soon Lily walked off with the rest of the first years, hurt, and Petunia waited in the ruined chamber. After some time, during which Petunia became frightened of being all alone in the dark ruin, a man in a purple robe came out of the great hall. He had a long grey-white beard and delicate half-moon glasses. As he approached her, he held up a letter and smiled.
"I'm glad we've found you." He said. "I just received this letter from your parents by owl. They're very worried about you. When they couldn't find you on the platform, they borrowed an owl from a kind wizard nearby on the platform. They'll be happy that you'll be returned safe, I'm sure."
Petunia glared at him. This was the man who had refused her letter request. "I don't even want to go here anymore! Who would want to go to school in this awful place? I don't want to be a freak like my sister. Who cares what she can do? I'm happy to be normal."
Dumbledore was astoundingly calm under this barrage of insults. He was saddened, knowing that jealousy had set into Petunia.
"Petunia." He said. "Don't let love be clouded by jealousy. Whatever small tricks we may conjure with our wands, love is far stronger. It is a magic beyond all we do here, and one that you need not be a wizard to practice. Remember that. Now, let's get you home."