I thought Petunia was a squib, but it was mentioned that Lily is indeed muggle-born so Petunia is just a muggle.
David Moulds for the News of the World - How does Aunt Petunia know about dementors and all the other magical facts she knows?
JK Rowling: Another very good question. She overheard a conversation, that is all I am going to say. She overheard conversation. The answer is in the beginning of Phoenix, she said she overheard Lily being told about them basically.
Is that true?
JK Rowling: Yes. The reason I am hesitant is because there is more to it than that. As I think you suspect. Correctly, but I don't want to say what else there is because it relates to book 7.