Your father's brother is your uncle. His wife is your aunt.
The second wife to your father if she is not your mother is called your step mother .
Matthew is either the son or step-son of his father's wife.
A person's father-in-law is the father of their wife or husband.
He is your Father in law.
He would be the father-in-law.
He would be the father-in-law.
Father in law
A father's mother to the father's children is called the Grandmother (aka: Nana; Gran, etc.) The father's mother to his wife is the wife's mother in law.
A father's mother to the father's children is called the Grandmother (aka: Nana; Gran, etc.) The father's mother to his wife is the wife's mother in law.
If your brother-in-law is your spouse's brother, then his father is your father-in-law. If you brother-in-law is your sister's husband, then his father is not your father-in-law, and is not related to you in any way..
Your father-in-law is your spouse's father, i.e. your wife/husband's father.