Relative turning up, is endlessly demanding (6)
I believe the answer is:
sister
'relative' is the definition.
(I know that Sister is a type of sibling)
'turning up is endlessly demanding' is the wordplay.
'turning up' is a reversal indicator.
'endlessly' means to remove the last letter.
'demanding' becomes 'stern' (demanding can mean stern or strict).
'is' written backwards gives 'si'.
'stern' with its final letter taken off is 'ster'.
'si'+'ster'='SISTER'
(Other definitions for sister that I've seen before include "Tender" , "A relative to resist" , "Resits (anag.)" , "habits' - one who has them" , "Female relation" .)