Is it permissible to use the amount designated for building a mosque—which the mother and sisters donated—to build a house for the widowed aunt and her orphaned daughters without the donors' knowledge?

It is not permissible to use the aforementioned amount for building a house for the aunt and orphans without informing the female donors. This is because either they intended for it to be spent on building a mosque, and you are their agent, and an agent does not act except with the permission of their principal; or they gave you the amount for a specific purpose, which is building a mosque, and the intention of the donor must be respected.