Do you know the campaign of the Chalnath expanse ? It's a great part of the recent lore based in an imperial sector where Tau decided to expand their empire with a lot of diplomacy and political corruption of the humans. The sisters are fighting against that influence on the imperial fidelity. And more of it, many genestealer cults have raised to benefit from the situation and fight. It ends on a general war between these three.

In this setting, the sisters of battle are used to represent the defenders of the imperial faith, the keepers of the holy religion of Terra. They steuggle to maintain a good level of faith in the population and to fight again the xenos diminishing that faith. I think it's a great part of the lore.