One of the Church's witches has ability to have a showdown in the souls, where the winner get the loser's everything, with ease of access said things depends on whether the loser gave up willingly or resist to the end.
Well, that witch 'ate' the 3rd sister, then eventually lost to Roland who uses the power of SCIENCE (by removing inertia, making the witch unable to do anything because she always 'slip')
Instead of gaining the knowledge and all that, the ability instead form a world where Roland and the defeated witch form the core of it.
All the losers of the show down became residence of that world, his 3rd sister and the defeated witch included.
The sister is just his neighbor and eventually help him out with understanding the 'rule' of this world.
The witch? She's now an albino loli and the last time she's mentioned she's learning how to protect herself.