Acquired relatives (2-4)
I believe the answer is:
in-laws
'acquired relatives' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for in-laws that I've seen before include "Relatives by marriage 2-4)" , "Christmas is when you might bid them welcome" , "Spouse's relations" , "A person's relatives by marriage" , "Family of your spouse" .)