It seems to me that all are agreed on a sale. The problem about which you can't agree is the method of sale. It will depend on the terms of the will as to what is allowable and what is not. However, the only way of truly testing the market is to place the property on the open market and see what a willing buyer is prepared to pay. Unless the will provides otherwise, yes the sister can force a sale on the open market - the executors have a duty to achieve the best price for an asset.
Are the sisters involved merely beneficiaries or are they executors as well - this can change matters under the self dealing rule.