Create a plan whereby you do the following: 

1. Pay a professional appaiser to appraise all properties. Let the siblings know you are doing this. 
2. Hire a trust attorney (run any plan past the trust attorney).
3. Once your plan is made, let the siblings know what you are doing. Let them know that you are not required to sell property to any of them (subject to any express directions in the documents), but if they want it they can get it at appraised value and if they are the highest bidder among the siblings. Let them know when they will need to make offers and have potential financing in place. 
4. Keep track of your time (you should pay yourself a fair rate for your time) and pay all expenses out of the corpus of the trust. 
5. Give intermediate accounting reports so they have an idea on costs and potential inheritance. 
6. Require each sibling to execute a waiver/release of claims before any distribution or property transfer is complete. (The trust attorney can help with this). Again, let them know at the beginning this will be required. 
7. Provide a full accounting to everyone. 

To answer your question directly: no distributions until after all expenses (including realtors) have been paid. If there won't be a realtor, then no expense for it. You could do intermediate distributions in the short term, but it would require some attorney help for intermediate waivers.