Have you heard about this? This theory from tumblr theorist sulietsexual about the truth of Aunt Petunia?
Sulietsexual theorizes that Aunt Petunia is actually a witch! And honestly, I am convinced.
In a interview with J.K. Rowling she states
“You might have got the impression that there is a little bit more to Aunt Petunia than meets the eye, and you will find out what it is. She is not a squib, although that is a very good guess. Oh, I am giving a lot away here. I am being shockingly indiscreet.”
The theory suggest that Aunt Petunia caused the breaking of Aunt Marge’s glass. Also highlights this scene in Harry Potter and the Deathly Hallows:
“For a moment Harry had the strangest feeling that she wanted to say something to him. She gave him an odd, tremulous look and seemed to teeter on the edge of speech, but then, with a little jerk of her head, she bustled out of the room after her husband and son.”
This could explain the unreasonable hatred that she developed for Harry. She resented magical people because she never got the opportunity to develop her own skills.