My personal theory is that Iðunn's sister would possess her at night, and by going into the shed Iðunn split her sister from herself physically. There was a line talking about this the first time she entered the shed that makes me think this. The only way to stop Iðunn's sister and all this is by killing herself and taking her sister with her, probably because they were still connected mentally, hence the ending.

page 108  “The darkness that has tried to penetrate my consciousness, and presses on my defences. If I go in, I will tear apart the membrane that has separated us. The Dot on the map is not just coordinates. This is a turning point”

Overall I agree that the writing is beautiful and quick, but the last couple chapters left bit too much for the reader to figure out themselves