You are talking about relatives, I sure can see problems arising here!
Yeah good luck!
Basically in baseball terms your mother inlaw in the team owner and you are the COACH; you get to tell the players how to act and call the plays but ultimatley your job security(as agent) is solely based on the land owner and how much authority they allow you to have.
1. Can the land owner still let people hunt with out asking you-Answer: Its their property and unless you have a written lease agreement they can do as they please with their hunting rights.
2. You will most likely be the first and last line of defense with people making dumbA$$ mistakes; shooting button heads, bringing unwanted or rude guests, filling out all the paperwork ect ect ect.
Best advice I can give is to sit down with your mother inlaw and get some CLEARLY defined rules and get each other on the same page.
THIS SHOULD BE A GOOD THING! - it all depends on how you handle it.