One sister, one child — as one (2,6)
I believe the answer is:
in unison
'as one' is the definition.
(I've seen this in another clue)
'one sister one child' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sister' becomes 'nun' (sister is a term of address for a nun).
'one' becomes 'i' (Roman numeral).
'child' becomes 'son' (male child).
'i'+'nun'+'i'+'son'='IN UNISON'
(Other definitions for in unison that I've seen before include "All singing the same notes" , "All speaking or singing as one" , "Simultaneously" , "All speaking or singing at the same time" , "All together at the same pitch" .)