[Deme]

It's a little vague from the scene itself...She's a witch and she's our witch, basically. We invent a position for her to do some witching for the good of Novan, while offering her protection against Lumen-haters. Why we don't do this for, say, Julianna, I don't know.
Maybe because Julianna's a) not our aunt, b) Already a Lumen on our side, and c) It probably wouldn't make her much happier (she seems like the sort to be too proud to accept protection easily), but may make Lucille happier.