Is it permissible to write a will dedicating all money after death to mosques and the poor, given that there is a brother, a sister, and a mother who are not in need of money?

It is not permissible for a Muslim who has heirs to bequeath all of his wealth, because his will is only binding up to one-third, and anything beyond that belongs to the heirs. There is no disagreement among jurists on this matter. If he bequeaths more than one-third, the heirs have the right to approve or reject the excess. As for spending money on charitable causes while healthy, there is no restriction on that.