What's the deal with Petunia?
I know that we're not done with her, but I can't guess how it will go.

Maybe one of her parents was a squib,
and a grandparent (Harry's great-grandparent) was "RAB."
And maybe he knew about the prophesy.
Rowling likes to take strange turns like that.

Nah. Regulus Black & Kreacher are still the most likely "RAB" suspects.