Is the death grant given by social security to the mother of the deceased considered part of the inheritance or her exclusive right? And is the son considered undutiful or sinful if he suggests to his mother and siblings that the death grant be allocated to building a mosque, while keeping the heirs’ shares intact?

A death grant, if it is a due entitlement for the deceased father because of his work, is an inheritance and should be divided among the heirs according to their prescribed shares. If it is a gift from social security, then it belongs to those designated by the granting authority. The questioner's suggestion to alleviate the burden from his needy siblings involves no sin or disobedience, and the mother's condition and consent should be taken into consideration when proposing it.