It just seems to me the sisters are being shortchanged. Their history and culture and varieties haven't been explored anywhere near as thoroughly as their Big Brothers (aka the Space Marines).
It's a pretty interesting story. Apparently they started out as a sisterhood of martial nuns devoted to Empy on an away and gone planet until Gorge Vandire decided that a bevy of shapely and violent female bodyguards was just what he needed. I'd blame Gorge for the curvy, clinging armor and perky little metal breasts, not to mention the high heels???? But apparently the Sisters of Silence were 'armored' in the exact same style. Maybe Empy was the Dirty Old Man who designed the prototype.
In any case the way I read it the 'Brides' were not only Vandire's bodyguard but his personal attendants and even entertained him with song, dance and 'more exotic practices'. I think we can all guess what that means. If what I suspect is true then Vandire was lucky Alicia Dominica settled for cutting off his head.
What really interests me is what happened when Alicia and her sisters were brought before Empy. Clearly it was a profound experience that affected them in different ways. Alicia seems to have applied herself to the reform and restoration of the Imperium - and made herself some political enemies doing it as she was killed by a traitor. Katherine is described as fiery and vengful ie: she liked killing things. Lucia was a more melancholic character. Sure she killed heretics and xenos too but she was also much given to penitential practices. Silvana's personality is not described but like Alicia she was probably murdered at the behest of an enemy from within the Imperium. Mina and Arabella seems to have been followers rather than leaders their Orders were founded in their memories rather than by them.
The Sororitas' extraordinary faith may be based on a special connection with Empy somehow handed down to them from the six founding Saints. A ritual initiation that somehow replicates their experience as they stood before the Golden Throne.