This is a tiresomely overworked phrase meaning "acquaintances and relatives." Kith and kin are not synonymous: kith means "friends, neighbors, and the like"; kin means people actually related through family ties, persons descended from a common ancestor. When properly used, both kin and kith are plural, so that one relative cannot be called kin and is not kith at all.
Dictionary of problem words and expressions. Harry Shaw. 1975.