That trickle of perspiration down Papa van Der Sloot's face during the interview gave the game away. Plus his rigid expression and almost total silence. Clearly they love their son, as any parent would, and my heart breaks for them. But I think the father suspects Joran is very much involved and he's scared to death. Still, the decent thing to do would be to make your son tell what he knows. Perhaps it was an accident, perhaps N struggled with J in the back seat, not wanting to be raped. He put an arm on her neck, maybe, and she suffocated. They might have thought at first she'd fallen asleep, but if they'd dumped her on the beach, she'd have been found by now. They should have brought her back to the hotel, dead or alive. They should have walked her inside, or if she was dead, they should have dialed 911 or its equivalent and told the truth. If they threw her body in the sea, that changes everything, of course.
I personally believe the father is involved, after the fact.