Relatives? Legally sons (2-4)
I believe the answer is:
in-laws
'relatives?' is the definition.
(I've seen this before)
'legally sons' is the wordplay.
'legally' becomes 'in law' (eg something may be the case in law or legally).
'sons' becomes 's'.
'inlaw'+'s'='IN-LAWS'
(Other definitions for in-laws that I've seen before include "Relations by marriage" , "Christmas is when you might bid them welcome" , "A person's relatives by marriage" , "Spouse's relations" , "Relatives by marriage 2-4)" .)