The answer is: be able to roll over before she will be able to sit without support
On average, proud parents tend to initiate many effort to make their infants more excel/advanced compared to another infants.
But that being said, they couldn't skip the necessary development that infants must pass before getting a certain advancement.
Rolling before able to sit is actually happen to all babies since during the earliest part of the development they still haven't develop enough control of their muscle to maintain the necessasry posture for sitting without help.
