Resist plan by relation (6)
I believe the answer is:
sister
'relation' is the definition.
(I know that Sister is a type of sibling)
'resist plan' is the wordplay.
'plan' is an anagram indicator (plan can mean organise).
'resist' with letters rearranged gives 'SISTER'.
'by' acts as a link.
(Other definitions for sister that I've seen before include "Nurse, nun or relation" , "Member of family" , "A relative to resist" , "Tender" , "twin?" .)