Happy to accept sibling who is compatible (10)
I believe the answer is:
consistent
'is compatible' is the definition.
(I've seen this before)
'happy to accept sibling' is the wordplay.
'happy' becomes 'content' ('content' is associated in meaning with 'happy'**).
'to accept' means one lot of letters goes inside another.
'sibling' becomes 'sis' (I have seen 'Little sibling ' mean 'sis' so perhaps 'sibling' could also mean 'sis').
'content' going around 'sis' is 'CONSISTENT'.
'who' acts as a link.
(Other definitions for consistent that I've seen before include "in agreement" , "Free from contradiction" , "Steady" , "stable" , "Unchanging in standard over time" .)