Ruthlessly incorporated elderly relatives (2,4,5)
I believe the answer is:
in cold blood
'ruthlessly' is the definition.
(I've seen this before)
'incorporated elderly relatives' is the wordplay.
'incorporated' becomes 'inc'.
'elderly' becomes 'old' (similar in meaning).
'relatives' becomes 'blood' (I've seen this in another clue).
'inc'+'old'+'blood'='IN COLD BLOOD'
(Other definitions for in cold blood that I've seen before include "Ruthlessly" , "Without feeling or mercy" , "Acting without mercy" , "with evil intent" , "snakes operate?" .)