Been bored at work, and was thinking about how Sisters fit into 40k. I started comparing the different armies to classic/fantasy archetypes. Maybe my Roll playing memories were just coming back to my thoughts.
Space marines - paladins
If I really have to explain this one, then you might be a bit oblivious. Space Knights/paladins, the stalwart defenders of mankind and their faith protects them.
Sisters - clerics
Less combat oriented than a paladin, but makes up for it by using Divine powers to sway the battle when needed.
Orks - barbarians
Brute strength; if it's not working, you're not using enough of it.
Dark eldar - rogues
Just taking a stab in the dark on this one.
Eldar - druids/shaman
Aspect focused, less direct combat powers/magic and more on the buffs and curses.
Necrons - wizards
Not direct combatants, They use powers far beyond normal peoples controls to do battle.
Chaos - warlocks
More combat oriented casters, often rely more on natural buffs and dark allegiances to gain their power.
Daemons - sorcerers
Bending the realm of reality to their will so they can fight through any means.
Tyranids - fighters
Usually do well with a healthy stat line, and a good combination of feats/skills to work their way through a fight.
Just keep shooting, just keep shooting. Shoot and move.
What does that leave us?
Guard - bard
uhm.... jack of all trades, master of none. Gives Verbal commands.... Inspiring presence. The normal man's man.
I think these actually fit pretty well. Obviously this isn't locking anything in place, there is overlap and not every unit in each army fits as well, but I think overall the army generally does have a play style that fits these.