But AS don't use potential for the hierarchy, just current strength. "One day you will stand above every sister, yada, yada."
I've tried to find something to say, but I've failed. The sister has many scenes, but the ones in which the detail could have been brought up with good effect are too easy to identify if I give any clue.
That would have to be a Kinswoman. I think a few studied with Sereille, and some were novices when she was an Accepted. Others were less lucky and were novices while she was Mistress of Novices.
I do seem to recall that RJ did mention one of them at some point, but I don't remember getting the full anecdote that's in the notes.