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.