Petunia always felt rather out-shone by her younger sister. Lily, being a witch, always was the 'life of the party'. She was always the topic of conversation. The family knew what going to ordinary school was like, but magic school was un-trodden territory for them, and they were eager to learn about the secret world they did not know existed.
Petunia, when raising Harry as well as Dudley, had never been very fond of her nephew. He probably thought that she hated him for no reason. But there was a reason. Petunia couldn't bear to look at Harry. When she did, her entire being filled with the regret of distancing herself from her late sister. And she couldn't look at Harry because he had her eyes. Those eyes that Petunia had not inherited from their father. Those eyes that had always shone with such enthusiasm and joy.
At Lily and James' funeral, Petunia only just held herself together. She cried for hours that night, and refused the company of her husband. She reminisced over the childhood memories she had of her sister. Now the only ones she would ever have.
When Petunia, Dudley and Vernon had to leave number 4, Privet Drive, she finally felt that Harry deserved to know why she had treated him the way she had. She regretted it, and it was selfish of her, practically ignoring the boy to spare herself the painful memories. But on that morning, she told Harry, that not only had he lost his mother that night at Godric's Hollow. She has lost her sister.