Would a son in law be a blood relative?

While a son-in-law is a relative, he is not a blood relative. Neither would be a daughter-in-law, step-father, step-mother, step-brother, step-sister, or the spouse of a 2nd marriage to one of your relatives.

Blood relative means another person shares DNA within a group of related persons. So typically, your direct genetic and genealogical (family tree) line would go from grandparent to parent to you. All of your grandparents' children (your parents and their siblings), your siblings, and your own children (when you have children) are blood relatives.

An adopted sibling would not be a blood relative to other biological siblings.