I have a married sister who always faints when she stands for Salah (Prayer); is she permitted to pray while sitting?
Standing is an essential requirement for the validity of the obligatory Salah. However, if your sister is unable to pray standing, she may pray while assuming a sitting position. This is based on the Qur'anic Ayah (verse) in which Allah (Exalted be He) says,
So keep your duty to Allâh and fear Him as much as you can
The Prophet (peace be upon him) also says,
When I command you to do anything, do of it as much as you possibly can.
There is nothing wrong with her praying while sitting in case she is offering a supererogatory Salah even if she is able to pray while standing. However, a person who offers a supererogatory Salah while sitting despite being able to do so while standing will receive half reward.