An interesting question is whether she feels obliged to wait until someone else makes the request. The purification of the Sanctuary didn't appear to need any outside trigger, but it's also not clear if this was passed down from the Night Mother, or decided by the Black Hand. Would an individual assassination require it? Are all the hits the player is asked to undertake initiated by the ritual, or are some (Permanent Retirement?) internal to the Brotherhood?
I think it was the Black Hand that decided it, wasn't the Night Mother actually aware of who the traitor was? Atleast it always seemed like it from the dialogue she says, but this is maybe just me interpreting too much into it.