I agree with the others that you should sit down with the family member and explain what all is involved and give them a rough estimate of the time that you THINK it might take, as well as an estimate of the cost of the fabric and supplies that will have to be purchased. Perhaps you can have them make those purchases directly, so you are not fronting your own money? Think about why YOU are doing this---is it something you want to do for your own personal pleasure or is it something you are doing primarily because the family member asked you to do it? Also, did you agree because (1) you want to 'help out' the family member, or (2) is your primary motivation an opportunity to make a little money on it? Both are fine, but if it is #1, the price you would charge would likely be a lot less than if your goal is #2.
But all that being said, I think an upfront agreement on the range of cost is REQUIRED to avoid an ugly family situation that could occur with a surprise.