Is the plan for the ones that want to farm, to farm together, or separately?  If the plan is to have them farm as one unit, then maybe they should start out that way and see how they get along.  Maybe rent them 25 acres or something on that scale.  That's how me and my 2 cousins are farming, only difference is, we bought our first farm ground together.  Each of our dad's own 1/3rd of the main operation.  Plan is down the road when we get enough land base is to split up the acreage, but then probably rent it back to the entity to keep it fair as long as we are using the same equipment, inputs, labor and such.  I'm just thinking outside the box making them work together.  It may help them decide if they want to go it alone, or with the help of their siblings/cousins.