Brusque, in sister's estimation (5)
I believe the answer is:
terse
'brusque' is the definition.
(thesaurus)
'in sister's estimation' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TERSE' is hidden in the letters of 'sisters estimation'.
(Other definitions for terse that I've seen before include "Brief; abrupt" , "Reset to be pithy" , "Curt among trees" , "Short and succinct" , "Curt, laconic of speech" .)