Unless the will otherwise provides, an administrator or executor with the written consent of the widow or widower and the heirs at law or devisees, or the guardians or conservators of such of them as are under disability, may sell the whole or any part of the real estate of a decedent, conducting the sale with fidelity and impartiality. The administrator, so authorized, may execute and deliver a valid conveyance of the estate to the purchaser. If there are heirs or legatees under disability, or whose whereabouts are unknown, or unknown heirs, the judge may appoint a guardian ad litem to represent their interests; said guardian ad litem to have authority to consent to a sale if, in his opinion, the best interests of his ward would be served thereby.