Is that son-in-law of your ancestor also his cousin by marriage? Is the Justice of the Peace who attests to your ancestor's signature on several documents the man of the same name who was his neighbor thirty years ago in a different state? Is your ancestor's first cousin also his third cousin on another side of the family? Just because two people are related in one way does not mean that they are not related in another.
Those relationships may be biological, by marriage, or through some other connection.
Never assume there is just one.