From a young age, Petunia learned that her parents favored her younger sister. Lily was more beautiful. Lily was smarter and more talented. Lily could make friends with just about everyone. To put simply, Lily was perfect in every way.
In complete contrast to her younger sister, Petunia was plain-looking at best. She got decent grades at school, but she never stood out in the classroom. Her teachers barely noticed her. Lily's teachers, on the other hand, were always talking about what a wonderful student Lily was and how sweet she was.
Petunia just never had Lily's confidence. Petunia was also quick-tempered, and she didn't make friends easily. But Petunia thought that she had every right to be so angry. After all, people were always talking about Lily, but no one ever talked about Petunia. It was as if Petunia did not exist. It was as if Petunia was invisible.
Then, to add even more insult to the huge pile that already existed, Lily was accepted into Hogwarts while Petunia was not. Once again, Petunia was not good enough. She just wasn't special like Lily was.
Their parents made sure that Petunia knew just how special Lily was. They were so proud of all of Lily's accomplishments, but they never seemed to care about any of Petunia's accomplishments. Petunia, after all, was just average and normal while Lily was absolutely brilliant and anything but normal.
It did not take Petunia long to become sick and tired of perfect little Lily.
Determined to get at least one victory over her perfect sister, Petunia started to refuse calling her sister by name. Lily was no longer Lily in Petunia's mind. Lily was merely her and she, and she wasn't worth mentioning in Petunia's world. Petunia liked to act as if she didn't exist. Petunia wondered how she liked being treated as if she didn't matter.
But none of this was ever much of a victory because no matter how hard she tried, Petunia was never able to forget that she would never be as good as her.