She was probably one of the kids at the temple in the order 66 scene. If she was hidden and saw Anakin being addressed by the clones as Vader then she’d know.
That’s what I thought too. I rewatched the beginning of Part 1 & the very first person we see is the little black kid we see training before Order 66. I have a feeling the show will end with the Third Sister being that kid and coming briefly back to the light to help Kenobi escape.