What does Aunt Petunia know?

[Lizra]

Please people...talk about HP!

My favorite part of OotP happened right off, when Harry's 2 worlds collided in chapts 1 and 2. Mrs. Figg was a squib, but even more goose bumpy was that Aunt Petunia knew what Dementors and Voldemort were!
I could see her knowing of Voldemort, he killed her sister. But why Dementors and wizards prison? She said "I heard --- that awful boy---- telling *her* (Lilly) about them --- years ago." Who was the "awful boy"?

Was Lilly's family strictly muggle till she came along? Or is there some wizard/witch history (skeletons in the closet) besides Lilly. (excuse me if this question might seem dumb ...but just answers will be great! )

Who sent the Howler???? I thought it was from Lilly...from beyond the grave, or something? I know that sounds dumb, but the howler said..."Remember my last, Petunia". But how could Lilly have given *last instructions* to her sister about Harry's care, when she was attacked by suprise and killed on the spot?

Just wondering..