Is it permissible to donate the remainder of the endowment’s revenue to charitable causes, such as the needy in Syria, in the event that there are currently no needy descendants, and is it permissible to spend from the endowment’s revenue on the grandmother and the incapacitated paternal uncle, given that there is no one to support them, and there are no needy descendants?

The default position, according to the majority of scholars, is to follow the condition stipulated by the endower (waqif) as long as it is permissible. If the endower specified a beneficiary for the endowment, then the disbursement must be made to that beneficiary.

If your grandmother is in need, then her maintenance is obligatory upon you, and it is not permissible to spend on her from the endowment unless you do not have other means to spend on her. In that case, you would be considered in need of fulfilling your obligation of her maintenance.

As for the uncle, his maintenance is primarily obligatory upon his siblings. If they do not provide for him, then it is permissible for you to spend on him from the endowment due to his kinship, in accordance with the Hanafi school of thought, which obligates maintenance for every mahram relative.

Regarding spending on those in need in Syria, there is no harm in it if you are certain that the children (of the endower) are not in need, and this should be done with consultation and mutual agreement among yourselves, as it falls under aiding and showing compassion among Muslims.

The preponderant opinion is that it is permissible to change the condition of the endower to what is more beneficial and serves a greater interest, as is the choice of Shaykh al-Islam Ibn Taymiyyah. This is because the stipulations of endowers are not like the texts of the Lawgiver in terms of obligatory application; rather, they are interpreted based on the endower's school of thought, custom, and language.