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.