I've got a kidnapped girl in the dungeon with rebel -17 after 8 weeks but I still don't have the option to brand her as a slave. at what point if ever is that made possible?If it's never possible, what purpose is there in kidnapping/torturing girls there?
You just ended up kidnapping a slave girl. Generally any girl with incredibly low rebellion is already a slave, branding a girl drops their rebellion like 30 points or so (From what I remember)
I believe girls you find in the catacombs may be branded as well. I haven't played in a while so forgive me if I'm wrong.