Family member tries changing by incremental amounts (10)
I believe the answer is:
stepsister
'family member' is the definition.
(daughter of a step-parent)
'tries changing by incremental amounts' is the wordplay.
'changing' is an anagram indicator.
'by' says to put letters next to each other.
'incremental amounts' becomes 'steps' (gradual steps).
'tries' anagrammed gives 'ister'.
'ister' after 'steps' 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" .)