Relation by marriage (8-2-3)
(Other definitions for brother-in-law that I've seen before include "Family member by marriage" , "Relative by marriage" .)
(Other definitions for daughter-in-law that I've seen before include "Female relative by marriage" , "One's son's wife" , "Woman married to one's child" .)
'relation by marriage' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sister-in-law that I've seen before include "Person related to one's spouse" , "Relation by marriage" , "Relative by marriage" .)