I think the key here is how close you are with the relative. Also relevant: to what extent is the relative genuinely benefitting from hiring you, and are you possibly being groomed to join a family firm?
So, some scenarios:
- the relative is a parent, older sibling, or someone who lives in your house or very near you and you see outside of work all the time. In this case, you can have a conversation with this relative outside of work. Don't say you're leaving. Tell the relative how you feel about not having any real work to do. Be accurate - don't say you feel guilty about being overpaid for what you're doing if the real issue is you're not learning and growing. Your relative may advise you to be patient and wait for something specific to happen that will let you start doing real work again, or may agree they are ok with you looking for more relevant work
- the relative is doing you (or some intermediary like one of your parents) a favour by employing you. If you don't need the favour any more, and can stand on your own feet, you can visit them at work, thank them profusely for "getting you started" and tell them you feel ready to look for different work now. They may ask you not to, and ask how they need to change the job to keep you (which will be an indication it wasn't a favour after all) or they may smile and nod and wish you luck and ask you to let them know when you've found something
- you are supposed to join this firm at a fairly high level some day. Use this "no specific duties" time to learn all you can about it. Are there inefficiencies in the mail room, the warehouse, the truck-maintenance, the floor-sweeping, that some day it will help for you to know about? What do the people at the lowest levels of the company do all day? What do the customers value? Who are your suppliers? Where are there pain points you may be able to address in a few years? Use your time to know this company as no-one else has ever known it. You'll be less bored, and when you ascend to your family-mandated place, you'll be very good at whatever job you have been given.
Your situation may fall somewhere between these extremes, but consider these options and mix-and-match to something that will work for you.
In the short term, if you have no boss, you could meet with the relative once to say that you really have very little to do. What the relative says then might help you to decide who is doing a favour to whom, and how much the relative wants you to stay with the firm. As well, if there is another relative between you (for example, if this relative is your mother's brother, your mother is between you), you could have this conversation with that relative, who may know the person and their motivations much better than you think, and may even have helped to arrange this situation.