[@Wraithblade6] They're from the Order of Our Martyred Lady, the same group that assaulted an Adeptus Terra scriptorum on the word of a single Inquisitor that they were mutants. The accusations turned out to be true, but that wasn't found out until After all the hundreds of scribes had been executed.

Currently, they are under the orders of a High Lord Inquisitor to 'neutralize' everything on the hulk, and your techmarine is standing next to one of the Emperor's Children, which are known as the chapter that went full-blown slaanesh cult on Terra during the siege of the imperial palace. Being as Dark Angels are now known as the chapter that drops everything to hunt traitors, especially their own fallen, and the tech marine isn't currently shoving his plasma torch into somewhere unpleasant on/in Vedius... well, Sisters are up there with Black Templar when it comes to slaying enemies of the empire.

So, yea... probably want to run from the scary nuns with flame throwers.