"Full" ownership of the property: meaning you own all the rights to the property from now into the future, is called "fee simple" or "fee simple absolute." When your father died, assuming he owned the property in fee simple absolute and gave your mother a "life estate" he transferred only a portion of the property; namely, the ownership and right to use the property during her lifetime. At her death, the property passes to the remainder beneficiaries. If you and your 6 siblings own the remainder interest in the property, then at your mom's death, you would probably each own a 1/6 undivided interest in the property.
In order to sell the property, the buyer likely wants to get a fee simple absolute interest, not merely purchase your mom's remaining life estate. Therefore, your mom should sell her life estate, and all 7 kids should sell their interest in the remainder interest of the property.
The purchase price paid by the buyer would be allocated among all 8 people. The value of your mom's life estate interest would be a portion of the fee simple absolute value of the property. That value could be determined by an appraiser, or perhaps by using actuarial tables of her life expectancy and multiplying those times the fair market value of the property (if held as fee simple absolute). The value of the life estate may be different for different purposes. For example, the IRS rules and regulations might impose one value, while a buyer might choose a different value of the life estate. You would need to check medicaid and estate recovery rules in NY to know how to value her interest for purposes of Medicaid. This is a complex area in which expert counsel should be sought. You might try NAELA to find an attorney who specializes in elder law.
THIS DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP. I AM LICENSED IN GEORGIA ONLY AND THE ABOVE IS FOR INFORMATIONAL PURPOSES ONLY.