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.