I think it's true that Ms Siska planned the whole thing. There is a weird poster in DreadOut act 1, It's say "REINCARNATE! Grand Assembly". Well I think its something like a group, who want the Serpent Mistress reincarnated. In ending of DreadOut Keepers of The Dark, Ms Siska wearing the same uniform as in that weird poster. So the point is, Ms Siska is a member of that 'Reincarnate Group' and her goal is awaken the Serpent Mistress.