The blue kitchen bowl. That seems odd to me. Perhaps used by the sister the last time she recalls slipping Jhessye food? Or maybe described by the sister as being used during the bleach cleaning frenzy?
as to the shredded possible phone book paper, I wonder if Jerice lined the closet with pages torn from a phone book. Or perhaps an old phone book was tucked in the closet and decomp fluids seeped into the pages.
As to the writings, I think AZlawyer is probably right. It is entirely possible that the sister kept a diary, a secret journal, or had a FB account she posted some things to. Children who have to keep secrets usually have a need to tell someone, even if it is simply a matter of writing it down and getting it out somewhere.
back to the yellow possible phone book shredded paper. A thought just occurred as I was typing that last. I wonder if little Jhessye, in her hours spent in the closet had a pen, or crayon, or something with which to write? At 5, being in school, she would have been learning to sound out words. It hurts to say but it is possible that this poor little princess was passing the time and writing or drawing on whatever was at hand in the dimly lit closet that was her prison.