What a brilliant set of character studies, Mystery Author! Petunia's bitterness, anger and spiteful resentment of Harry and all things magical now makes perfect sense, as does the utter spoiling of Dudley. Neither Mrs. Evens nor Petunia are particularly admirable people in many respects, but it's easy to see the open wounds left by a less than loving mother and how the damage done made Petunia what she is; she gets our sympathy legitimately. How awful for her that all of her sacrifices seemed to go unnoticed and unappreciated by her mother, and how equally awful that in an attempt to hide her sister's 'abnormality' from Vernon, she cuts off her ailing mother's means of communication with her sister and triggers the depression which may have caused her mother's death. How horrible of Mrs. Evans to have played favorites in this way, taking one daughter completely for granted while behaving as if her other daughter was all that mattered; how dare she mourn herself to death over the seemingly prodigal daughter while ignoring everything the daughter who never left her has done for her.
And into this toxic milieu steps a white knight, and what Petunia sees and loves in Vernon has never been so clear. He *is* a perfect gentleman to her and one can feel her soaking up his single-minded attention the way a rose overgrown by weeds would soak up the sun after they'd been cleared, would feel more of those weeds being cultivated away the longer she was with him, would feel as if she was finally getting something of her own.
It's also good to see a version of Lily who has faults, and who can be as willfully blind as any other human being. That she would show up in her mother's room in such a way, heedless of the commotion she's causing and the display of magic in front of a Muggle she doesn't know and then vanishing makes me understand Petunia's resentment very well. Truly excellently done!