Is the financial amount that used to be disbursed by Social Security to the deceased mother, and is now disbursed to the son who was added with her, a right of the son or of the heirs?

The social security funds belonging to your mother after her passing are for all the deceased's heirs, unless the social security system stipulates that they are transferred to the youngest son. In that case, the agency responsible for social security must be consulted. If it is confirmed that the funds are rightfully due to the heirs, then your brother must return them after deducting his share. If he does not have the money, it remains a debt upon him, unless the heirs, or some of them, forgive it. One must be cautious of consuming unlawful wealth and should be aware of and abide by the established regulations.