Actually, Scooby Dee is Scooby Doo’s sister, as stated in one episode where all three siblings, Scooby Doo, Scooby Dum, and Scooby Dee get together. Scrappy Doo is Scooby Dee’s son. One imagines that Dee was her married name, but still that leaves one to wonder why Scrappy was Doo. Of course, it’s still to expect continuity in a cartoon.