First off, you want your family to enjoy themselves on the wedding day, not to be working…while it sounds great to save money by having them do things, think about what they might miss.
Second, where is the line drawn between guest and professional? For example, we recently did a wedding where the couple’s brother acted as the DJ. He had DJ’d in the past, had the equipment, and best of all (at least in the couple’s mind) he worked for free. Problem was, during the wedding he acted like a guest and neglected many of his DJ duties and there were a lot of complaints.
In another wedding we worked, the mother, mother in law and grandmother did all the cooking….they ended up being servers instead of guests and by the time the dancing started they were too tired to participate.
In most cases you are better off thanking your friends and family for offering, but let them know you want them to enjoy your big day and not work it…the little you save in cost is quickly lost in the experience of not having them enjoy themselves.