Therein lies the problem - since your relative has obviously incurred a "debt" your service was in fact anything but. "free." Sooner or later one of you is going tofeel short changed.
Go with Val's advice - do it for free and then be done with it.
I guess it might be hard for some to understand -- but that's pretty much the way things work in an Italian family (and circle of friends). I don't think it's as much of a debt, as it is a courtesy you are extending to someone you care about. It's not required that they pay you back in some way -- it's a voluntary thing.
I think here's a good example:
Grandma and my Aunt lived on the farm -- I went there for every holiday; ate lunch there almost everyday most of my life. So I was living down here in FL, and the family was throwing a surprise party for my Aunt's 70th birthday. I hopped on a plane, since my gear was still in NY, and decided to do the gig -- nobody asked me to, I just wanted to.