Apparently there is a "semi-secret" (whatever that means) society named the Brothers of the Seven. It was established by Ironbriar, Vorel Foxglove and five other merchants in Magnimar about a hundred years ago. I can't see what they actually do (except for some blackmailing, extorsion, and some real-estate financing).
What is the Brothers of the Seven and what do they do?
One of the cult's leaders (Ironbriar) also leads a second cult called the Skinsaw Men (worshippers of Norgorber). This is actually a cult of murderers that comes from old blood - a master cult in the city of Vyre, although it is now very much its own entity.
It actually says that Ironbriar established the Brothers of the Seven as a cover for the Skinsaw Men cult. Why would you use one illegal organization to cover another? That doesn't make much sense to me. And what kind of cover are they supposed to provide? Is the cover just the lending the Seven's Sawmill? You would hardly have to establish a semi-secret organization just to operate a Sawmill...
Also, the Sawmill is called the Seven's Sawmill, and is supposed to be a cover and a safe place to meet for the Brothers of the Seven. However, it seems only Skinsaw Cultists work there and visit sermons.
Are All Skinsaw Men also members of the Brothers of the Seven?
All in all, the two names seem to be used interchangably and this can be confusing. For example, on p. 40 it says "Magnimar's guards aren't equipped to handle a cult as crafty and sneaky as the Brothers of the Seven "- when talking about the Skinsaw Men's murder spree.