My mother has just passed away. We are three children, but we have not heard from one of my brothers for many years. Can the estate be settled without him?

No. Dividing an estate requires the participation of all heirs. The notary will suggest hiring a genealogist to locate your brother.
If these efforts fail, the guardianship judge may, at your request and with supporting evidence of your brother’s disappearance, issue a judgment of presumption of absence (Art. 112 Civil Code). This allows for the representation of the missing person and enables the estate to be administered.