Ok... Try this...
Nicki is right. Twins are possible. Moreover:
not all => some are (just not all).
=> there ARE twins (Squishy also alluded to Nicki having a point)
=> 9, 2, 2 or 4, 3, 3
Then using my earlier pseudo-logic - 4, 3, 3 not possible/likely
=> 9, 2, 2
Also a 9 year old is more likely to play the piano (though there is no reason why a 4 year old couldn't)