My late sister (died of SIDS at 10 months) had Prader-Willi.
I would be shocked if a child could make it to 9 years of age without a diagnosis- it's a very severe condition that has a lot of physical features, and almost always mild to severe mental retardation.
That said, there is a connection between genetic PW (impacting the #15 chromosome(s)) and autism, but the research is still a work in progress.
When PW kids "are always hungry," it means "they will beg, borrow, and steal to eat. The kitchen usually needs to be kept under lock and key, and PW kids will eat non-food items like pillows or paper." There are also common facial characteristics in PW people- again, I'd be very surprised if your cousin has it.
That said, PW and autism have some similarities, and there's no reason someone can't have both autism and PW.