Consanguinity: relationship by blood, from Latin sanguis, for blood.
Affinity: relationship by marriage.
Affine: a relative by marriage. “In-law” is relatively recent; it is not in Merriam-Webster Collegiate Dictionary Third Edition. Not too long ago, “sister-in-law” referred to your brother’s wife or your wife’s sister. It did not include the more recent usage of a spouse’s brother’s wife. For example, I refer to my wife’s brother’s wife as my brother-in-law’s wife, not my sister-in-law. My sisters-in-law are my brothers’ wives and my wife’s sister.
Agnate: related through male descent or on the father’s side.
Cognate: related on the mother’s side.
Much confusion exists about the definitions of cousins, especially the use of “second cousin.” First cousins, also called cousins german, are the children of siblings. Second cousins are of the same generation and are the children of first cousins. Third cousins are the children of second cousins, and so on.
Even more confusion exists about the term for a person’s relationship to the child of his cousin. That person is often mistakenly called a second cousin, but the correct term is first cousin once removed. They are related through a first cousin and are one generation removed. The grandchild of your first cousin is your first cousin twice removed, and the great-grandchild of your second cousin is your second cousin thrice removed. The second cousin usage can confuse people even more because dictionaries often report usage without guidance as to the correct term.
Pres. William McKinley was the second cousin of my great-great-grandmother Rachel Badger because their parents were cousins. They were descended from David McKinley, their great-grandfather, who served in the Revolutionary War. Rachel is four times removed from me, so Wm. McK. is my second cousin four times removed.
A term I hate is “direct descendant.” I am the descendant of Rachel Badger and of David McKinley. I know of no other kind of descendant. “Direct” is redundant — how could I be an indirect descendant?