father
His nephew .
It is correct if you are talking about more than one nephew. If only one nephew is involved, it is nephew's father. My nephew's father is my brother. I am talking about one specific nephew. If you mean to refer to the father of your nephews, then it would be nephews' father.
The English language recognizes no family ties between your nephew and your father-in-law.
Your father's great nephew is the son of your father's niece or nephew. Your father's nieces and nephews are your first cousins. Their children are your father's great nieces and great nephews, and are your first cousins, once removed.
Your father's brother is your uncle. You are his niece (if you are female) or his nephew (if you are male).
You would be a neice or nephew, neice if a girl and nephew if a guy.
To your uncle, you are his niece or nephew.
His father
Lot, the nephew of Abraham, was the father of Moab.
nephew
Depends whether your father is dead or alive. And, there seems to be some family "issue" here, ie why has your nephew got your father's will? If your father is alive and he has made a will then it is no business of yours or anyone's until he dies. So, in this case forget it! If your father has died and you cannot get his will from your nephew then, if the will has been probated, go to the Court where that was done and ask the Court for a copy of it. Some more information would still be helpful here though. And, the obvious answer to your query (whn i first saw it) is "Ask for it from your nephew" or "Ask your father for a copy of that which he has given to your nephew".