For as long as Petunia could remember, her little sister had copied her. "Tuney" did it first, and Lily did it second.
It was normal, supposedly.
Little sisters admired their older sisters and did whatever they did (supposedly).
Petunia supposed that everything would have been fine and normal if not for one fact. When Lily did something that "Tuney" did first, she always did it better.
Lily was the pretty child, she was the favored one. She had the gorgeous red hair which all the neighbors commented on, and loved. She had the unique green eyes, and the cute little face. She was smarter, faster, prettier, more daring, and more outgoing than "Tuney" ever could be.
Because Tuney did it first, and Lily did it better second.
Tuney would be the first and last to ever admit she was jealous of her sister. Petunia would acknowledge it straight out to herself, in her mind, but to the rest of the world (to the neighbors) she was so proud of her adorable little sister. And that was how it always was and always went, even after her and her sister fell out of favor with each other (in private, of course, couldn't let the neighbors or her—God forbid—friends see).
And then, finally Petunia did something first, and she did it better.
She had Dudley—and Dudley was perfect! Much, much better than that horrible child her sister had afterwards.
It didn't matter that Dudley was slightly overweight and a little unhealthy, even though Lily's child was healthy and skinny in a good way. It didn't matter that Dudley learned less fast, or processed things slower, or wasn't as intelligent. He was perfect. Because he was Tuney's.
But there was still only one problem: Tuney still did it first, and Lily still did it second—even if Lily's child was a million times less as good as Tuney's own.
So when that dreadful child of Lily's ended up on her doorstep, the opportunity to be better for good presented itself to Petunia.
Because she knew what her sister would do if it was opposite—if Dudley was on Lily's doorstep. She would raise him and love him and provide for him as much as she would her own child. She would care, and help him, and do everything possible, along with mourning the loss of her sister (even if they had fallen out).
But it hadn't happened that way, and it never would.
So when the time came to raise and love and care and help Harry, Petunia didn't do it.
She did the opposite.
She didn't raise and love and care and help little Harry Potter, instead she neglected him.
And Tuney was happy. Because she did it first, and there was no way Lily could do it after—she couldn't even do it better!
And so Petunia denied mourning her sister, denied Harry, and denied the feelings inside her that felt strangely like guilt.
Because Tuney was first, and Lily wasn't second.