A family member is turning severe? Not entirely (6)
I believe the answer is:
sister
'a family member' is the definition.
(I know that Sister is a type of sibling)
'is turning severe? not entirely' is the wordplay.
'turning' says the letters should be written backwards.
'severe?' becomes 'stern' (I've seen this before).
'not entirely' means to remove the last letter (most but not all of the word).
'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 "Resist (anag) - nurse" , "trade union member?" , "Member of family" , "Nun's title" , "habits' - one who has them" .)