She is young.
Nissa is described as very strong in the One Power, and Egwene al'Vere would like her to be raised to Aes Sedai before the Last Battle. It means that she is among the new recruits by the White Tower, because only in recent years Aes Sedai were able to find so strong channelers. It indicates also that her minimum level of strength is superior than the one of Lelaine, Elaida or Romanda. Nissa could be at the level of Cadsuane or even above; usually women at the level of Lelaine or Romanda spend 3 years as Novices and 3 as Accepted but Nissa (as Nicola) was Novice for a very short time, almost an year, or even less.
When Egwene and Silviana return, Egwene tells Nissa and Nicola that they have done very well, and gives them more responsibility. She allows them to leave Tar Valon, and sends them to the Traveling ground to weave a gateway to Caemlyn. They are to retrieve ter'angreal from Elayne Trakand, who is expecting them. The ter'angreal are used for entering the World of Dreams, and Egwene tells the girls that she is going to begin training them and some others in their use. She explicitly tells them not to use the ter'angreal without her permission, and sends some soldiers along on their errand to keep the pair in line.
When Silviana questions why Egwene did not swear Nissa and Nicola to silence, Egwene says it is because she wants them to brag about being trained with the ter'angreal. Egwene hopes this bragging will reach the ears of the Black Ajah, who will then be tricked into thinking they know where to find her.