Thanks for the mentions @Matt Devincenzo!
@Eric Fiedler first we need to see the Deed or vesting document. These grandchildren may not even have an active interest. They may just have some sort of interest that kicks in upon the death of someone else. Who knows?! You need to start with figuring out exactly who owns what and how.
Everyone is pointing you in the right direction from a legal perspective, but honestly, I think you were warmer with your idea of just buying the other guys out - depending on how much the pain the $#! guy is going to want. Again, look and see who owns what and how; then you may be able to buy them out for what you would end up spending on a partition action.
The thing with a partition is just as Bill G mentioned; it's not a guaranteed thing, can be a long process and the parties may not end up with property in the end anyway. Second, you have no ownership in the property so you can't bring this partition action. Who is going to file the case? Certainly not the pain in the $#! collecting rents. Sounds like he has no problem with the current arrangement. Are the others motivated to sue?
Last thought...you mentioned the 1/2 interest guy is someone the grandmother owed money to. Did he have a lien against the property; like a mortgage and note? Assuming the 1/2 interest wasn't enough to satisfy the loan, you could purchase the note and foreclose.
Either way, you need a real estate attorney to look this property up and tell you exactly who and what you are dealing with.