Prayer: Uncertainty About Who the Eldest Son Is
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1373
If it is not known who the eldest son is, it is not obligatory for any of the sons to perform their father’s qaḍāʾ prayers. However, the recommended precaution is that they should divide the qaḍāʾ prayers between themselves or draw lots (qurʿah) for performing them.