I can't find the source, but I read about a study done with women where they smelled sweaty shirts from unrelated men or their brothers or fathers. Across the board they rated the shirts from their blood relatives as smelling worse than the others. The theory was that people with different genetics have different makeups of microbes living on their skin, making them smell different than you.

I like the way my wife smells, and we certainly share no relatives for many, many generations.