Informally, a relative by marriage (2,3)
I believe the answer is:
in law
'relative by marriage' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for in law that I've seen before include "Wife's mother" , "as counsel will advise" , "relative term, maybe" , "one brought closer with Act of Union" .)