What is the correct course of action for the son regarding his mother's will, in which she bequeathed money to his brothers and his deceased sister, and how should the monetary amounts be distributed in the presence of his father and his two younger brothers?

A bequest to an heir is not valid unless all the other heirs approve it, due to the saying of the Messenger of Allah, peace and blessings be upon him: "Indeed, Allah has given every rightful person his due, so there is no bequest for an heir."

Therefore, if the mother made a bequest to her children, and this is a bequest to an heir, and the rest of the heirs did not approve it, then the bequest is nullified, and the inheritance is distributed among all the heirs according to their prescribed shares, including the husband and the deceased sister (her share is then distributed among her heirs).