Ultimately son, you fear, is like sister? (5)
I believe the answer is:
nurse
'sister?' is the definition.
(thesaurus)
'ultimately son you fear is like' is the wordplay.
'ultimately' says to take the final letters.
The final letters of 'son you fear is like' are 'NURSE'.
(Other definitions for nurse that I've seen before include "Angel of mercy" , "Nanny" , "Foster" , "One ministering to the sick" , "Who cares" .)