Your son-in-law is your daughter's husband.
The father of a son-in-law is typically referred to as the father-in-law.
You call your son, or child whom is age 5 and he will figure it out. LOL You call your son, or child whom is age 5 and he will figure it out. LOL
I would think you would call her by her name, same as if she were your son's American mother-in-law.
Your nephew
daughter in law
daughter in law
Your nephew
Either nephew or nephew in law.
Your brother's son-in-law could also be called your niece's husband, but he is not related to you.
daughter...daughter in law.
Son in law
Your cousin in law