Many of the best harmonizing groups are siblings/families. There's something at work there that goes deeper than musical talent and skill -- there's something genetic deep down in their bodies and minds that just goes together.

How could I have forgotten to mention the Everly Brothers? Then there's the Wilson brothers and cousins (Beach Boys), the aforementioned Roche sisters and many many more.

Martin's original post, however, seemed to be more about the ability to improvise harmony as an *individual* skill. Some have it, some don't, and I believe that it's a skill that must be practiced regularly to remain strong. If you know how to harmonize, it matters little whether the singer(s) with whom you are harmonizing are your relatives -- there's just a little extra intangible quality there if they are.