In distress, she's visibly upset (6)
I believe the answer is:
sister
'in' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sister that I've seen before include "Nurse, nun and sibling" , "of the same origin" , "trade union member?" , "Close female relative" , "habits' - one who has them" .)