Petunia?!
Okay..if you want to hear my theory (but maybe it's a bit far fetched...)
Mark Evans is a wizard and he'll attend Hogwarts next year as he'll be 11 then. I mean, he must be come up later, oserwise I don't see the point mentioning him (and calling him as Harry's mother) at all. And if he was a wizard his parents could easily be wizards too. And we all know that Petunia usually doesn't talk about magical relatives.
I agree with Kristina about the fact that he'll be a first year at Hogwarts next year. I mean, JKR doesn't do these things for no reason. There are people that are mentioned once or twice and then turn out to be really important. I just wonder why Harry was taken to live with the Dursleys instead of a wizarding family that he's also related to. I guess we'll find out though.
Lily comes from Muggle parents........BUT that Mark Evans kid could have ALSO been sent to a foster home or something IF something had actually happened to his parents when he was a child. Then of course Harry had to go to Petunia b/c she is a blood relative! 😄 Mark Evans could very well be in the same boat as Lily...another witch in the family! 😄