Much of my work is based on kinship theory, mostly from the field of anthropology.
A site by Tim Roufs -- "Cultural Anthropology" (University of Minnesota at Duluth) -- has good basic definitions for kinship terms and groups. It has an index to Marriage, Kinship, and Descent. There are no doubt many others with this type of information, but this is the first one I came across when I was looking for good general information. I'll add more as I find them.