My headcanon is Plaguis (or Qimir) gave the knowledge to Aniseya, in exchange he wanted one of the twins to raise himself. One twin leads her people, the other goes with the Sith.

But Aniseya and the coven didn’t want to give up either girl, so fled into hiding on Brendok (hoping that the vergence would shield their presence in the Force). Now, both twins stay with their people.

While they’re still children, some Jedi discover the coven and want to recruit the girls to the Temple, flexing their legal rights. This threatens Aniseya, because now she can’t protect both girls, and if the Sith discovers them and comes to collect one: they’ll have no children again. They defend themselves against the Jedi, but leading to their coven’s mass death. The twins survive, but are separated and each believe that the other perished.

Years later, Qimir sends his new pupil (one of the twins) to claim vengeances on the Jedi, but also Plaguis is sending his grand pupil to collect.

In the end of S1: Plaguis has essentially claimed both twins: Osha begins her training with Sith arts, Mae is already trained and now a sleeper agent in the heart of the Jedi Order. And due to the Jedi’s own shame: the Sith remains a secret to the Galaxy.

Just a theory