An uncle.
The son of your parents is your brother.
If your aunt is the sister of one of your parents, her husband's brother (her brother-in-law) is not related to you. If your aunt is the wife of a brother of one of your parents, then her husband's brother (her brother-in-law) is either your father or another one of your parents' brothers.
Call your social services.
The daughter of your brother or sister is your niece.
brother or sister
Any children of your parents' siblings are your cousins.
The brother of your brother is also your brother. Brothers are males who have the same parents. If they have only one parent in common, because there was a divorce and remarriage before one of them was born, then they may be called "half-brothers."
Yes, your mother's mother's brother is commonly referred to as your granduncle or great-uncle. This term denotes the brother of one of your grandparents. Alternatively, you could simply call him your uncle, although that is more commonly used for the brother of your parents.
If you mean in the case of one of your parents re-marrying - the children of the new partner are your step-brother/sisters.
You still call your brother, your brother.
There is no name for such a person, because there is no relation. Example. Bob and Susie are brother/sister. Susie marries Tom. Bob and Tom are Brother's in law. Bob has NO relation to anyone else in Tom's family. Susie does however.
you pin them down till your friends parents call them