What would the outside world know about the Sewn Sisters?

My players had a lengthy conversation with Nanny Pupu, who I set up as having an antagonistic relationship with the Sewn Sisters, but who made a deal of non-interference with them. Because of this deal she couldn't talk in specifics, but did give my players a number of (generally accurate) broad theories about how they might be behind the Death Curse and why. This did mean they learned the name, the Sewn Sisters, and what they are, a coven of hags. They don't know any fine details of what they're doing, other than gathering souls somehow.

When they went back to Port Nyanzaru they gave a generally accurate report to Wakanga, including that. And it does seem like Wakanga might invest some time in investigating the name. It's not clear what he might find. I see this as an opportunity to give them some lore/background on the sisters and build them up as antagonists some. Does anyone have suggestions for this?