Purity: Obligation of Covering Private Parts from Others
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 53
It is obligatory (wājib) for one to cover his private parts when emptying his bowels and/or bladder, and at other times, from people who are duty-bound (mukallaf),1 even if they are his maḥram,2 like his mother and sister. Similarly, it is obligatory for one to cover his private parts from an insane person and from a child who is mumayyiz, i.e. someone who is able to discern between right and wrong. However, it is not necessary for a husband and wife to cover their private parts from each other.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
[1] A mukallaf is someone who is legally obliged to fulfil religious duties.
[2] A maḥram is a person one is never permitted to marry on account of being related to them in a particular way, such as being their parent or sibling.