I agree requiring a contract for a family wedding may be considered tacky. But it is worse than tacky to ask a family member to do such an important task for free. Whoever asked for this favor clearly didn't have the photographer's interests as a guest in mind. It means, essentially, that the photographer be dis-invited as a wedding guest and employed for the event much like a bus boy or parking valet -- except for no pay. There is no way to enjoy the festivities and company of family when saddled with so many responsibilities.
Besides, I doubt this contract issue would come up. Anybody with enough courage to ask for a contract from a close relative would have enough courage to decline the "opportunity" in the first place.