Demand from vain sister (6)
I believe the answer is:
insist
'demand' is the definition.
(to insist on something is to demand it)
'from vain sister' is the wordplay.
'from' says the answer is hidden in the clue.
'INSIST' is hidden in the letters of 'vain sister'.
(Other definitions for insist that I've seen before include "Urge" , "Take a firm stand" , "Demand determinedly" , "Demand forcefully" , "Put one's foot down" .)