@Opelia That's the power of experience over the childishness. Btw Aria is 14 and Mielle 13 years old (edit: at the start of the novel).
Also I think Mielle is quiet since she thinks she's won, when earlier she was only getting blow after blow.
Mielle can't carelessly make her ex-naive sister do things she did in the past, and she simply has no brains for anything bigger. xD