Legally, it's relative (2-3)
I believe the answer is:
in-law
I believe this clue is a double definition.
'legally' is the first definition.
(eg something may be the case in law or legally)
'relative' is the second definition.
(I've seen this before)
'it's' is the link.
(Other definitions for in-law that I've seen before include "Relative by marriage" , "Near relative" , "Spouse's relation" , "Meghan to Harry's family?" , "Family member, by marriage" .)