In all likelihood they won't be able to appreciate what the service really costs. I did something for my sister once telling her I would charge her for materials only. She refused to believe the materials cost what they did and I took a loss on the job rather than argue with a family member.
If you are in a financial position to provide things for your family even if you don't get paid appropriately, and want to do so because you love them, go ahead. If not, then tell them you are not comfortable doing work for family. If you try to straddle that line you will find yourself with family friction and neither your work nor your family will be a happy place for you.