There's a lot of great advice above. However, most of it is centered around preserving everyone's financial interest. The part of the discussion that is lacking is emotional side of things and the relationships.
More than twenty years ago, I was placed into a similar situation (parents owned a 200+ acre farm/ranch with one lane of egress, my brother and his wife wanted to live there, etc), however, my parents never asked my opinion and just gave them an acre and the small old house/garage that was on it. I got nothing. At the time, it really didn't bother me, as that was my parent's decision and I wasn't concerned about fairness. As long as everyone was happy, that was good enough for me.
For many years, life was great for all involved. They all lived on the farm, they grew closer together, and a mutual beneficial relationship developed (as both parties need assistance from the other). Then everything changed overnight. There were accusations of my bother/his wife not respecting my parent's portion of the property (inviting guests to go hunting on it without notification), allegations of spying with telescopes, hiding behind bushes, and showing up at each other's houses unannounced, etc. The tension got unbearable and both parties tried to draw me into their side of the fight. I tried to remain neutral (as best I could) but everyone got mad anyway. The situation got so bad that my brother/his wife would not speak to my father for many years. The night before he pasted, I pleaded with them to come visit my father (literally right next door) as my father knew his time was up and he wanted to reconcile. Fortunately, that happened, but just barely.
However, before he passed, at a time when he was still upset/untrusting of my brother, my father changed his will and my mother's will so that I was the sole executor the estate. I was also granted durable power of attorney too. I think that devastated my brother and caused an irreparable fissure in our relationship. Although I refused to go along with any changes that would alter the 50/50 distribution of the estate and I promised to seek my brother's concurrence for any POA actions that I may take, it didn't make any difference. It's now been nearly 7 years since that happened and my brother still doesn't communicate much with me, this despite my constant attempts to keep him fully informed on my end. Very sad...
The point of all this rambling is that while you can't predict what is going to happen, there is a chance that the relationships could go sour. With both stuck on the same property, there's no room for a retreat by either party. The old saying "fences make good neighbors" applies here too. We all need boundaries in our lives. When you tear them down, bad things can happen.
I highly recommend the book "Boundaries" by Henry Cloud. It speaks volumes about this kind of stuff.
