Together — one with sister and one with child (2,6)
I believe the answer is:
in unison
'together' is the definition.
(I've seen this in another clue)
'one with sister and one with child' is the wordplay.
I cannot really see how this works, but
'one' could be 'unison' (I've seen this before) and 'unison' is found in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
A single letter 'i' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for in unison that I've seen before include "All speaking or singing at the same time" , "All together at the same pitch" , "All singing the same notes" , "All speaking or singing as one" , "Simultaneously" .)