niece in-law
A cousin, in all those difficult cases they are your cousins
Your nephew's wife is your Niece-in-Law, whether she is caring for a child from a previous marriage or not. Few people would care to be that precise, however, so most will probably just call her your Niece.
Your wife's nephew is your nephew, but the English language has no relationship term for your nephew's wife. Just call her "my nephew's wife."
Your sister-in-law is either the wife of your brother or the sister of your spouse. The children of your brother's wife are your nieces and nephews, since they are also the children of your brother. In some cases they may be your sister-in-law's children from a previous marriage. Then they are not related to you at all. However, if they are very young, it may be convenient to call them your nieces and nephews anyway. The children of your spouse's sister are always your nieces and nephews.
Second cousins once removed.
your nephews
Nephews
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
No. Your wife's niece is also your niece, but the husband of the niece is not related to you or to your wife.
your niece's or nephews
Dads typically call their sons by their first names or nicknames.
if they are the children of your brother, they would be your nephews and nieces - if they were her children before she married your brother, then step-nephew and step-nieces