The sisters of battle are the brave warrior girls of the Imperium of Men. They always watch over the endless populations of human worlds with a keen eye for the future heretic.
There are also some cases, were advanced prove of loyalty is required. Meaning they seek out shy guys, who have never felt the touch of a woman. The sister will then demand, that these boys become men. It usually resolves in several rounds of bonding, which enhances ones readyness to do your duty, no matter the cost. You will then be a hardened citizen, ready to face the many hardships of life and get physical, if required.
If they are on the battlefield and can't find young boys, they just fuck the guardsmen.