The most common and most likely answer to this crossword clue is the 3 letter word NUN
I've seen this crossword clue in The Times
N
U
N
More clues leading to the result NUN
- News about union leader’s sister?
- News about union leader’s sister
- One who has taken vows? Not any, some say
- Sister hiding in kitchen unit
- Joshua's father's sister
- Religious female going backwards and forwards
- Sister nobody’s spoken of
- Member of Order — note posh name
- Sister in cotton uniform
- Sister having nothing to say
Buddy explains!

The straight part
We know that "Sister" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sister = nun.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NUN mean?
NUN noun- A member of a Christian religious community of women who live by certain vows and usually wear a habit, (specifically) those living together in a cloister.
- (by extension) A member of a similar female community in other confessions.
- A prostitute.
- A kind of pigeon with the feathers on its head like the hood of a nun.
- The fourteenth letter of many Semitic alphabets/abjads (Phoenician, Aramaic, Hebrew, Syriac, Arabic and others).