This is not a family law question. I will move it to the Probate and Real Property sections so you will get some more answers. Good Luck.
Tough situation since all the porert is not inthe probate. The PR can only sell one half.
I think you are going to have to do a partition action and get a court to compel the sale since 25% want to stay (two siblings) and 75% want to sell (two siblings and the estate).
In short, you will need to sit down with local counsel and see how best to handle it. It is just a little odd when you have a will that compels a sale of the property but only controls half of it.