The sisters repentia have some historical precedent. There were "naked warriors" in various bits of history, like the celts occasionally fought the romans naked. Freaked the romans out, at first. Then they realized the gaesatae may look disturbing running at you naked with weapons in their hands and their dorks hanging out, but when the romans stopped running and started fighting them, they got their naked asses handed to them.
there were ancient greece era warriors, etruscans,who would go into battle with breastplates at times but with their twigs and berries handing out. Sensational but not that effective either.
So yeah you can argue human history has naked warriors freaking out the enemy. How freaked out an ork, tau or god forbid a tyranid is gonna be at the site of a nude woman is..... debatable.. Some orks might be kinda disgusted. "Oi! Ain't doze oomies gots no decency?! Yer don't go inta a fight all wiffout clothes like dat! Ain't proppa.."
A tyranid might be thankful it doesn't have to spit out the clothes when it's done chewing.
I suppose naked guys running at eldar might demoralize them, coz you know human coupling gear has got to be bigger than what those boney snobs have...
Necrons might stop a minute and think "Hey, didn't I use to have something like that?" if attacked by naked guys, and "Hey, aren't I supposed to be feeling something about now?" when attacked by naked women.
But in "reality" a sister repentia is a bullet riddled corpse before she gets ne ar any competent, armed enemy. Yeah, her boobs might be big but that eviscerator is a lot bigger and I know which one I'm going to be focused on.
if you wanted a practical sister repentia...how about one with a set of armor on the front half only, and on her back she's got a honking suicide bomb? Before going into battle she loads up on some holy joy juice that ramps up her metabolism and makes her ignore pain and injuries that don't actually take off parts. She runs into an enemy formation and no sooner than you can say "For the emprah!" sets it off. Instant redemption, go directly to the emperor, collect 75 handsome virgin studs, enjoy the afterlife.
Edited by Professor Tanhauser