jealousy can make people say and do horrible things: here I drew two stories for petunia, one where she didn't become a witch, one where she did. (characters belong to J.K. Rowling's Harry Potter series)
The really awful thing is that, even with her sister dead, witch!Petunia is probably a lot happier than Petunia Dursley. I mean, in canon we never see Petunia to have any real friends, she's completely consumed by bitterness and I don't think we ever see her properly happy. Witch!Petunia probably has a few friends and connections made at Hogwarts, no reason for bitterness and would probably be able to find some bittersweet happiness in raising her sister's child, none of which are even possible for Petunia Dursley.
I think the artist is working with the theory that if Petunia had become a witch, she would have never met Vernon, and even if she had being a witch she wouldn't have met his standards for being "normal" and so would never have married him. Making it impossible for the Dudley we all know and love to exist in the same form as he does in the original series
At least in story #2 Harry isn't considered the unwanted. Sure, Dudley wouldn't have been born... or would he have been? *thinks about that* But either way, there wouldn't have been a redemption of sorts for Dudley near the end.