Robert,
I guess you have never heard of a partition suit. This is fairly common in inherited properties where one or more of the owners do not wish to sell. Those that do wish to sell can bring a partition suit and the judge will most often order the property to be sold instead of a physical split and the procedes split between the owners.
In the worst case scenario the parties can sell their interests in the property to anyone that will buy the interest and the remaining owners will find they own the property with someone they do not even know.
Talk to a lawyer in Washington state.
In a case like this, it just takes one of the owners to stop the process and you have 5 people involved.
At least here in California, you cannot even get the home listed without the signatures of all parties.
I would guess your going to have to spend some quality time with the other owners and really pin down what their interests are. See if you can satisfy them so that you can get the go ahead to start the process to sell the home.
R