A sibling's unchanged (2,2)
I believe the answer is:
as is
'unchanged' is the definition.
(I've seen this before)
'a sibling's' is the wordplay.
'sibling' becomes 'sis' (I have seen 'Little sibling ' mean 'sis' so perhaps 'sibling' could also mean 'sis').
'a'+'sis'='AS IS'
(Other definitions for as is that I've seen before include "in the present circumstances" , "current state" , "not changed" , "In present condition" , "unaltered" .)