What must I do in the case of discussing with my father about waking my siblings for Fajr prayer; must they perform the prayer upon the Adhan, or is it permissible for them to delay it as long as the time of sunrise has not entered?

It is obligatory to perform the prayer before its time expires, and it is preferable to perform it at the beginning of its time, right after the Adhan. The jurists agree that the worshiper has a choice in performing the prayer within the extended time, whether at its beginning, middle, or end, and there is no sin upon him for what he chooses. Prayer is permissible at any part of its time, and there is no validity to the notion that it must be performed at the very beginning of its time. Therefore, do not be harsh with your siblings if they perform the prayer within its time, especially if they are young and not obligated to perform the prayer in the mosque. However, if they are obligated to perform congregational prayer in the mosque, then explain to them and to your father, with gentleness and wisdom, the virtue of praying in the mosque. Do not anger your father, for angering him is an act of disobedience to parents (Uquq), and the Prophet, peace and blessings be upon him, said: "The Lord's pleasure is in the parent's pleasure, and the Lord's displeasure is in the parent's displeasure."