While we haven't heard much from his family, we also haven't heard that they aren't cooperating. I would question any family or friend needing my vehicle in the middle of the night. If they did question him, RF may have just said it's an emergency and that he will explain later. Or, he may have lied and said he had a flat tire/ran out of gas/dead battery. I'm not defending anyone who may be withholding information. I just think the media and LE have led us to believe they are withholding info when they may not be - or they may not have any to offer.
If RF is responsible for Zuzu's disappearance, he's surely capable of lying to his family.