I think I might be looking at this from a different angle. You say this is a family member, and they must be fairly close family members to invite you up to see their new log cabin!! I am sure she has seen your quilts and would absolutely love one for the new log cabin. However, even though she sews, she probably has no idea how much time and work goes into a quilt. That was one thing that got to me, when I started doing quilts. It was not quick. I did not see much progress as you do with sewing. Even though I am currently sewing a wedding dress for my daughter, I will have it finished much faster than I would a quilt. There is the great misconception. Most sewers will not realize the time factor.
So, I would explain to her the time factor and that you are in the middle of several projects. However, you would love to spend some time showing her how to make her own quilt. You could start off by teaching her just the bare basics, and do a table runner. She will either love it, or hate it. But she will have an idea of the work that goes into it. She might even be really interested in learning to do it herself. Not everyone will decide to do it, and just take off on their own. I just figured when I started if I can make dresses, pants, etc...I can surely sew a bunch of pieces of fabric together.
Its not nearly as easy as that !! But honestly I can hardly wait to get done with this dress, and get back to my quilting.
What ever you decide, she is family and I hope you find a solution that will satisfy both of you. Good luck.