Effortless adoption thus returned second sibling (7)
I believe the answer is:
osmosis
'effortless adoption' is the definition.
'thus returned second sibling' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'returned' is a reversal indicator.
'second' becomes 'mo' (both are short periods of time).
'sibling' becomes 'sis' (I have seen 'Little sibling ' mean 'sis' so perhaps 'sibling' could also mean 'sis').
'so' in reverse letter order is 'os'.
'os'+'mo'+'sis'='OSMOSIS'
(Other definitions for osmosis that I've seen before include "Something like dialysis" , "Passage of bodily fluid through porous partition" , "Process of gradual absorption" , "Diffusion; assimilation" , "Diffusion through a semi-permeable membrane" .)