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.