Disclaimer: I don't own Harry Potter.
Imagine Petunia growing up knowing that she was the unwanted child. Maybe because it was because of her looks, maybe her parents had deeper reasons. Who knows? Humans can be petty. While there are mothers who would die so that their children could live, there also are mothers who would throw their children away. Anyway, whatever reasons the Evans couple had, young Petunia felt that she was unloved. And that was what made her surly and irritable.
Lily was her total opposite. A pretty child, the Evanses, even Petunia, doted on her. Some would say it was because when she did good things for Lily, her parents would smile and approve of her actions. That was partly true. Petunia really did shower her love on her baby sister because Lily was the only one in the family who loved Petunia back. There was a time that Lily adored her big sister and Petunia was proud of it. During the first few years of Lily's childhood, she and her big sister were inseparable. That was until Lily met that Snape boy and started hanging out with him more and more. Petunia couldn't get Lily to see only her older sister once more. It certainly didn't help that Snape's barbs against Petunia whenever they fought (only when Lily wasn't around) were about how ugly and untalented she was. How could she ever think that her sister would be interested to still talk and play with her after finding someone who was as magical as she was? Petunia's heart did not break when Lily got that letter to that magic school and left to study and live there most of the year. It did not break when Snape had somehow found out that Petunia had written the Headmaster to ask if she could go to the magic school, too. No, Petunia's heart broke only after the Evanses minus Lily came back to their house in Cokeworth after their trip to King's Cross Station to send Lily off to school. Left alone with their parents, who were bereft over the loss of their golden daughter, Petunia became a non-entity. She was once again alone and unloved.
Whenever Lily came home, Mr. and Mrs. Evans really didn't approve of frog spawn in her pockets and teacups turning into rats but they were just so happy to have their lovely daughter back so they ignored the strange things. Although they couldn't understand half of their beloved daughter's stories, they smiled and doted on her just like in the old days, wishing somehow that if they could just show her how much they loved her, she would revert to being a normal girl and stay with them forever. But those wishes never came true. Instead, they were left with an ugly, uninteresting child. If only Petunia were the one who was a witch, the one who went to that far-away boarding school, the one they saw less of. Of course, the couple didn't say these things out loud and they never physically hurt Petunia. They provided her food, clothing, and shelter, so in their minds, they thought they were doing okay.
Petunia, for all her surliness, never had any of Lily's bravery. She never stood up for herself and questioned her parents why they were treating her like that. She accepted their mere tolerance of her and bore their unreasonable behavior. She instead blamed magic for taking the one person who had loved her away from her. She and Lily couldn't get along during the holidays because Lily could never stop talking about magic and school, while what Petunia wanted was to talk about her life to Lily. The problem though with having been doted upon by the family (and friends and admirers) was that Lily thought the world revolved around her. She was ignorant and intolerant of her sister's grievances. Never having experienced neglect from her parents, she couldn't believe Petunia's complaints and instead thought that her sister was simply jealous of her abilities, which was also true; Petunia did wish that she was a witch and wondered what her life would have been if she were one. By the time Lily had grown out of that mindset, too many things had cropped up to make healing the rift with Petunia a priority. Mr. and Mrs. Evans deaths occurred, Lily got involved in her magic world's war and new people came into the sisters' lives. Then Lily died and of course, by then, it was already too late to say sorry.
Author's Notes: This was first published on my Tumblr page.