I think not, based on [spoiler]Petunia’s remarks in SS: “How could you not be, my dratted sister being what she was? Oh, she got a letter just like that and disappeared off to that — that school…I was the only one who saw her for what she was — a freak!” Doesn’t sound like someone who had magical aspirations herself. Much more likely she just wanted to be valued for what she was. Which would explain why, as an adult, she’s so fiercely borgeouis and conservative: if she can only be ordinary, she’ll be the champion of ordinariness.
And why doesn’t Harry know his maternal grandparents, anyway? Did Voldy kill them too, or did they die of non-magical causes?[/spoiler]
Well, we’ll just have to agree to disagree on the first point. On the second, either Rowling will let us know at some point, or she overlooked the matter entirely. At any rate, the senior Evanses must be dead, or they would be in contact with the Dursleys. First of all, Dudley is their grandson too, even if he is a brat, and second, they wouldn’t turn their backs on Harry, because he’s the last link to Lily. They might have agreed to go along with Dumbledore’s plan to give Harry an underprivileged childhood, but complete absence must mean they’re no longer living.
Harry’s grandparents must be dead or else Petunia wouldn’t be his only living relative.
I don’t know why I’m going to bother to use a spoiler for this BUT
Elaborating on the earlier theory that Petunia and Lily had a brother who was the father of Mark Evans, then Petunia could have been Harry’s only living relative if Mark’s father died while Mark’s mother was pregnant