As far as i am aware her DD will have a seat of her own and i am assuming she will have her youngest DD in a carrier or in her arms (only a couple of months old).
That's great! All I can suggest are light toys, colouring in, books to read her, and make full use of the in flight entertainment . How long is the flight? We've done Brisbane to Singapore, Singapore to London, London to Glasgow, Glasgow to Cork, Dublin to London, London to Singapore, Singapore to Sydney, Sydney to Brisbane, Brisbane to LA, and LA back to Brisbane. DD1 was under 2 though so we didn't get a seat.