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.
Yes, niece and nephew-in-law are valid terms.
A son is a blood relative. An honorary son is a boy that has been welcomed into a family and its thought of and treated like a real blood relative.
Yes, most people consider a son-in-law, the husband of a daughter, to be related to them. That is why, at or just after weddings, a son-in-law may be "welcomed to the family."
He is your nephew, and you are his uncle.
Yes, most people consider a son-in-law, the husband of a daughter, to be related to them. That is why, at or just after weddings, a son-in-law may be "welcomed to the family."
No. Your son-in-law's brother may be a son-in-law if he is married, but he is not your son-in-law.
I'd use her name. If the Brother in law is married to your sister, she would be your nephew's wife. And you are not related by blood.
There is no actual blood relationship. You would typically introduce her in that way, My son-in-laws's sister. Many refer to her as the sister-in-law.
If your sister-in-law is the wife of your brother, she is the mother of your brother's son. Her son-in-law would then be the brother-in-law of your brother's son. If your sister-in-law is your spouse's sister, her son-in-law is not related to you or to your brother's son.
Your son-in-law's brother is your daughter's brother-in-law but is not related to you.
The singular of "son-in-law" is "son-in-law." It remains the same in both singular and plural forms.
If your sister-in-law is the sister of your spouse, her son is not related to your sister's son. If your sister-in-law is the wife of your brother, her son and your sister's son are first cousins.