Is the estate of deceased parents to be divided according to Sharia, or according to their wishes (the mother's will stipulated a specific division)?

If the parents did not divide their inheritance during their lifetime, then what they said is considered a will, and a will to an heir is not enforceable unless approved by the rest of the adult, mature heirs. If they approve, it is permissible to act upon it. If they desire the legal division, they are entitled to it, because the Prophet, peace and blessings be upon him, said: "Indeed, Allah has given every rightful person his right, so there is no will for an heir."