Since you have found that the market price for the coop hasn't dropped that much, why not try to come to an agreement with your brother to put it up for sale now, knowing that it may not sell at all, or it may take a while. Also, include that you will rent it out to maximize everyone's income from the property while it is on the market. Try to come up with a bottom line figure that is acceptable to sell it for amongst your brothers. Don't clue in the realtor how low you are willing to go. You want to make sure that you don't end up with another conflict when someone throws a lowball price out there and your brother wants to grab it. If you approach him with the idea, and quickly get to the subject of price and ask him what he thinks should be the lowest offer they should consider, you can hope that he comes up with a high enough figure that's workable for everyone, and then you have it in your back pocket that it was his idea not to go below this certain price in the first place. If his idea of a price is too low, then you need to market the coop to your brother in hopes of getting him to come up with another number on his own.
Luckily, when I went through this, I realized that my sister was more important to me than squeezing every last dime out of things. I discussed the issues with her up front, and told her that I was going to try to handle almost everything and would not make a move on anything without letting her know what I was doing first. She let me go on without argument with most things, and the few items that I sensed she had other ideas on, I simply conceded immediately and did what she wanted. The only thing that I didn't handle this way was the house. She wasn't ready to jump on selling it quickly and I told her that the housing market is going to take a big dump and we need to sell it now. She is quite pleased that she listened to me now, since we got $1000 over our listing price, had a contract signed by the first person that looked at it the day it listed, closed in about 17 or 18 days, and the listing prices in the neighborhood started dropping significantly within the next few months. I was also able to get pretty much top dollar for furniture that we had no use for by selling it to the people buying the home. The furniture was just too nice for most homes in the area, and we both have fully furnished homes. We would have ended up getting nothing out of it otherwise. But the main point is that we came through it without an argument.
I realized how important this was. My cousins (they're brothers) haven't spoken to each other in over 30 years. One of them died last month, and the other one feels awful that they let settlement of their parents' estate come between them and he has finally realized what a big mistake it was after all these years. I did not want the potential to be there for this to happen with my sister.
Anyway, good luck with the situation. Maybe you can get your brother out for a drink alone some night and find some common ground.