Ooh, tough to disagree with the DIL. She will think you're just taking your son's side.
I struggle with the "copying" issue all the time. We get most of our inspiration from seeing things others have done, at least I do (not much originality here!) But - even if there weren't the issue of copyright, I just don't think it's that much fun to make a true copy of someone else's work. If we truly use it as a springboard for our own creation, we get much more satisfaction from the effort. That's why we quilt.