Is the endowment mentioned in the question considered legitimate (Shar'i), and are the heirs entitled to claim the value of the rents that were spent on charitable deeds and that the paternal aunts took from the inheritance?

For a waqf (endowment) to be valid, it must be immediate, unless it is contingent upon death, such as when one bequeaths that their property become a waqf after their demise. This type of waqf is considered a will (wasiyya) and is valid according to the majority of scholars. It is restricted to one-third of the estate and cannot be made to an heir unless the other heirs consent. If the grandfather did not make the waqf immediate but instead made it a will to take effect after his death, then it is a will restricted to one-third of his wealth and cannot be made to an heir (such as the paternal aunts) unless the other heirs consent. The son has the right to object to anything exceeding one-third or to the bequest made to his sisters.