No sale of real estate hereunder shall be made unless all persons interested in the real estate to be sold (which shall be the full fee simple title in the tract sold if the possible claims of the person or persons not in being affect the full fee simple title thereto, or the full undivided interest in the tract sold if such possible claims affect only such undivided interest) are made parties to said proceedings; provided that where the real estate to be sold is subject to a mortgage or other lien, the mortgagee or lienholder need not be made a party to said proceedings if the sale is made subject to such mortgage or lien. Where the person or persons not in being who may claim an interest in the real estate sold belong to a class of which there is a living member or members whose interests do not conflict with those not in being, such living member or members of said class may be made parties plaintiff or defendant and may appear on behalf of themselves and the unborn members of the class, but in every case the court shall appoint a disinterested person as guardian ad litem for such person or persons not in being, and such guardian ad litem shall be required to file a written answer or other pleading fully disclosing the possible interests of such unborn persons and take all appropriate steps to protect their interests.