Pleased about sister agreeing (10)
I believe the answer is:
consistent
'agreeing' is the definition.
(thesaurus)
'pleased about sister' is the wordplay.
'pleased' becomes 'content' (associated in meaning).
'about' means one lot of letters goes inside another.
'sister' becomes 'sis' (synonyms).
'content' placed around 'sis' is 'CONSISTENT'.
(Other definitions for consistent that I've seen before include "the same as always" , "Unchanging in standard over time" , "uniform" , "stable" , "Maintaining a standard over time" .)