Is a grandmother's verbal will to the questioner's mother and her children valid, stipulating that they take her mentally challenged sister's money in exchange for caring for her, provided that the grandmother's son does not object to this will?

The will to disinherit a mentally disabled daughter is invalid, even if her mental state necessitates guardianship. The guardian may not take anything from her money except out of extreme necessity and only to the extent of the need. The appointment of a guardian is made by the father or the ruler, not the mother.