The son of your parents is your brother.
The daughter of your brother or sister is your niece.
Your son or daughter. Just because the child gets a new set of parents, doesn't mean that the birth parents cease to exist. Adopted children have two sets of parents and both parents will call that child theirs.
Sometimes its because the parents don't want their son or daughter calling all the time and bothering the other parents.
Your first cousin is your son's first cousin, once removed. Since that cousin's sister has the same parents, her relationship to your son is the same - first cousin, once removed.
If the son is a minor and the parents have custody, yes. If the son is not a minor and has told the lawyer not to talk to his parents, no. The son is the client, even if the parents are paying. If the son does not mind, then yes.
Your son will be a nephew to your brother, and will call him "Uncle".
You do you call your sons's son your "grandson" !
You would call the son of a lord, John, son of Lord So-and-So
It does not really matter whether it is the groom or bride's parents who call. In many cases the groom's parents call, but, etiquette does not dictate this.
Your parents god son is not technically anything to you, however some would consider their parents god son to be their god brother
I'd normally call him by his first name. And the son of your aunt is your first cousin.
The English language has no term to describe a relationship between you and the parents of your daughter-in-law. In English-speaking cultures, you are not related. If you have to refer to them other than by name, you can call them "my son's parent's-in-law" or "my daughter-in-law's parents."