Since the numbers of posts of the theory thread don't work anymore because it was moved, i'll leave my theories here, which are just my way to see the whole game and all it elements. Maybe it's a summary of all the theories made before, but i've waited until i could see the ending of the game to make up mi mind about them.
Sabitsuki might have a disease called "Hermansky–Pudlak syndrome", just because she seems to present the same symptoms of it. (To me, she is not sick of Cancer). This illness can be detected because of the hemorragic events that may occur, and also, it is visible because of the albinism, which that people suffer from their birth.
Sabitsuki maybe found herself in the flow state (psycologicaly talking, i read that a long ago) looking for a better way to live with her sickness, and that's why she gets all these effects, which mostly alterate her body.
The Uterus area is, to me, the Sabitsuki's mother's uterus (the way she imagines it). Sabitsuki was born with that disease, and The three headed "monster" looks like corrupted sperms (and freaking scary). Maybe Sabitsuki consider herself one of these defectuous sperms, because she was born with that disease.
Since i saw the original art by lol, i might presume that Smile and Sister are related to her, those are her brother and sister. Maybe they were a family of three children. (See the Three head monster, which can be related to three sperms) Sister is afraid of her Sabistuki because of her disease. Smile maybe tried to comfort Sabi for some time, but when they went to school, he left her because he considered their friends much more important. The one who made her "smile", laughted at her. After that, Sabitsuki moved alone, and lived like that for some time, totally isolated.
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