Yes, your spouse's nephew is your nephew-in-law.
Yes, your spouse's nephew is your nephew-in-law.
Either nephew or nephew in law.
Yes, niece and nephew-in-law are valid terms.
either your niece's husband, or your nephew, or your nephew in law, etc
Your brother-in-law is your spouse's brother. His grandson s your spouce's great nephew. (Some people say grand nephew). That makes him your great nephew, too. Your brother-in-law could also be the husband of your sister. Then your brother-in-law's grandson is your sister's grandson and your great nephew.
The English language recognizes no family ties between your nephew and your father-in-law.
Nephew-in-law! However, most people leave off the in law but in technical terms, it is nephew-in-law.
your grand niece or nephew in-law (-Kasadie)
If your brother-in-law is your wife's brother, his son is your nephew and you are his uncle or aunt. If your brother-in-law is sister's husband and his son is also her son, he is your nephew and you are his uncle or aunt. If the brother-in-law's son is from a marriage to a woman other than your sister, and your sister has not adopted him, he is not related to you. However, you may choose to treat him as your nephew, especially if he came into your life as a young child and considers your sister to be his mother.
either your niece's husband, or your nephew, or your nephew in law, etc
You and your wife's brother's son are related through marriage. You are the uncle-in-law to your wife's brother's son, and he is your nephew-in-law.