Disclaimer: I don't own Petunia Evans/Dursley. Also, some of this doesn't quite fit in with canon, but it wouldn't have worked otherwise. Please enjoy.

Petunia Evans had been twelve years, eight months and six days old when Lily had received a knock on the door from the-man-in-the-purple-cloak. Now, of course, she knows that he isn't that, he's Dumbledore for goodness sakes, but she was, she remembers, only young then, and so the-man-in-the-purple-cloak he was.
Mummy and Daddy had thought he was mad. She had too, but Lily had seemed strangely…accepting? No; unsurprised. Tuney had thought she'd imagined those times from when she was little – she thought she'd imagined the fallen branch and Lily flying off the swing – but now she realised that it wasn't just the heat of the summer. Lily was a…a witch.
It was fun to start with. Lily had gotten a list of things to buy, and the two sisters had stayed up late together, giggling over the idea of toads and cauldrons and wands, and the man, who was Dumbledore by now, said he'd take them shopping. But then Lily'd invited 'Sev', and then they'd gone, and then Tuney was left alone, trailing behind them, window shopping the world she'd never know.
And then Lily left.

Lily had always been thinner than Tuney, and, she guessed, that was probably why she'd started purging. She'd eat and eat and eat with Mummy and Daddy, and they'd ask if she was okay, and she'd say yes, but always with that voice. You know, the voice that means, no I'm not. But their eyes were distracted and their minds were elsewhere, and Tuney would get up quietly and leave the table, and then she'd be in the bathroom with the tap on loud and she'd be sick and sick and sick and sick, and then she'd wipe her mouth – and her eyes – and then she'd flush the chain and she'd go to her room and do her homework and tidy her room like a good girl.
When Lily came back for Christmas, her pockets were full of surprises: she had so many things to show them that Tuney almost found it fun, but at the back of her mind…at the back of her mind she was jealous, a little bit. So she'd go to her room and do her homework and clean and purge and cry, but she didn't complain, not yet.

When Tuney was sixteen, she was one of the first girls in her school to get an A* in every subject. She'd worked really hard for it, and she'd been really proud of herself, and she'd laughed and smiled and wondered (just a little) if maybe Mummy would be proud too. Clutching her precious slip of paper, she'd run home in the sunshine, her long blonde hair streaking behind her, and she'd breathlessly nearly fallen through the door and she'd shouted … but no one replied.
She had cocked her head, 'just like a dog' she'd smiled to herself, and listened. But she couldn't hear anything.
Finally she saw them. In the garden. That…that boy, he'd brought Lily an owl. An OWL. So Mummy and Daddy and Lily were playing with it, but that was okay, she reasoned, Mummy would still care, right?
So she'd gone into the garden with her paper, and with shiny eyes like stars she'd held out the sheet.
And the owl had eaten it.
Tuney had cried and cried and cried, and she'd shouted 'freak! Take your freaky thing away!' and then Mummy and Daddy had shouted, and she hadn't known what to do, and she ran up to her pretty, tidy room and cried and cried and cried.

Lily wasn't Lily any more, Lily was the-freak. Petunia missed Lily – she missed Tuney too, for that matter – but she understood now that some dreams belong in childhood, and sisters-being-friends was one of those things. So she went to a party with her friends, and there she met Vernon. He wasn't actually very fat then – he 'did weights' at the gym – and he was the first person in a very long time to laugh with her and comment on her smarts and tell her she looked pretty, and that was, it seemed, all Petunia had needed.
They had a whirlwind relationship and he swept her off of her feet, and what did it matter if Mother pulled odd faces when she brought him over, and what did it matter if he was arrogant – that Potter boy that the-freak was swooning over was cocky and Father didn't say anything about him now did he. So when Vernon got down on one knee, in the park, surrounded by the heavy scent of lilies, Petunia took one fleeting look at the gaudy flower bushes around her before settling her gaze back on Vernon. 'Yes'.

Their household became a nightmare soon enough. Petunia would make meals for both of them, always, but once one is out of the eating habit one may find it hard to reacquaint oneself with food, and so Vern would eat it all, and then have his smelly friends in her lovely house and they'd get drunk. She didn't like it, but Vern was so sweet to her when he was on his own, and he was working so hard to provide for them, who was she to complain?

Dudley was born soon enough, and she loved him more than she had ever loved anything before. He had her curly blonde hair and Vern's pale blue eyes, and she must admit she did spoil him a little. But for a while, everything was perfect; Dudley was the magic they needed to bring them back together, and who cares about the food and who cares about the drunken friends and who cares about the dirty footsteps on your clean carpet when you and this man made this perfect, golden little boy?

And then one day, she woke up to a baby crying, a baby whose eyes promised her that she would never be able to forget her perfect little sister. And it all crashed down from there.