Crossword Genius

One sister, one child — as one (2,6)

Ross

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" .)

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