The golf clubs should fit - and I would imagine luggage for 2 adults should also fit. The kids keep their luggage with them - on floor near them. I agree that 4 sets of golf clubs and luggage for 4 will not fit in back.

That said, our kids, boy and girl 4 years apart, are not especially close - but on vacations they morph into loving brother and sister - I think without peer pressure to act superior to your sibling and the absence of other kids makes that more likely. (Part of the reason we like family vacations - even now that they are 14 and 19)