Naughty priestess capturing the heart of brother's relative (10)
I believe the answer is:
stepsister
'relative' is the definition.
(stepsister is a kind of relative)
'naughty priestess capturing the heart of brother's' is the wordplay.
'naughty' indicates an anagram.
'capturing' indicates putting letters inside.
'the heart of' indicates the central letters.
The central letter of 'brother' is 't'.
'priestess' anagrammed gives 'sepsister'.
'sepsister' enclosing 't' is 'STEPSISTER'.
(Other definitions for stepsister that I've seen before include "member of the family" , "relative in law" , "Relation gained through a second marriage" , "one who hated Cinderella" .)