How can I see in the game the nuns with the different outfits ?
Go through the game, spy on them.
Yes I did that but I was talking about their sprites. Somewhere I saw different versions of their dresses and I'm pretty sure I saw it at some point in the game but I can't remember when (and I finished all the existing quests)
They were in different outfits in the first quest in the monastery, when they were waiting for the priest from the witch.
