There's currently a bug (not sure if anyone noticed it) but everytime you choose the Terror Sister or Terror Osiris, the game completely stops working and doesn't let you do anything, either fight, go back, etc...
Heres the proof about them:
#Nightmare Co-op "Terror Sister" and "Terror Osiris" Softlock
bleak drum
lyric dragon
sullen schooner
Still broken. 
hybrid trellis
was just about to post this as well