Remember that the Adepta Sororitas (Sisters of Battle) have a double duty to both the Ecclesiarchy (as men are forbidden from defending the church) and the Ordo Hereticus (the Inquisition). The Inquisition uses the Adepta Sororitas as their primary military arm. Owain gave some ideas for distinguishing them on the tabletop, but sounds like you are looking for a storyline to give your Sisters some background. I personally field an all-mech force of Sisters and have used that as a reason to give them a tie-in to the Mechanicum of Mars (if only we could include tech-priests in a Sisters force). Consequently, though they must heed the call from Inquisitors and the church, they are more aligned with the Mechanicum and their background is vastly different than the Orders mentioned in the Codex. Alas, I have no name for them as of yet (I keep vacillating from one name to another). But you could come up with just about any background you can imagine, however, Adepta Sororitas are always at the call of the Inquisition / Church, and due to their very nature are faithful to the Emperor. Hope that helps
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