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Note: I've had this idea floating around for quite a while and finally got around to typing it up. And I must say, I absolutely love the new(ish) Copy-N-Paste method of uploading stuff. Please leave me a review if you read. It's easy to do and it means a lot to me.
Lily never really understood Petunia.
Lily was the more practical child, for all that she was younger. While she read books about dinosaurs, Petunia read about dragons. When they went on a nature hike at the park, Lily identified nearly twice as many plants as Petunia, because Petunia was poking through the flowers looking for faeries.
If Lily sometimes did something unusual, well, she just sort of assumed that Petunia couldn't do those things because she was too busy to bother figuring out how. Just like how Petunia could whistle and Lily couldn't be bothered to learn. And anyway, Petunia was busy with silly stuff like drawing pictures of unicorns and castles to hang up over her bed.
Petunia loved magic. But she didn't have any.
It was Lily that got a Hogwarts letter.
Petunia never forgave Lily.