This had a very good story buildup. The language is beautifully written with only a few minor grammatical errors here and there. I liked the varying choices between the siblings, which could risk wasting time and causing the rot to spread even further. The characters were all beautifully described, and I think the themes of what each character represented fits exceptionally well with the theme of the game jam.
I also appreciate your little message at the end of the game. I could really see this as some sort of small animation of sorts or even a book series. And, if given more time for art and sound effects, the game could have an even bigger impact from the visuals. But I understand that the primary focus of this jam was the writing part lol. And I think you succeeded with that very well.
My only suggestion is putting parts of your story, especially the introduction, into smaller, more digestible pieces for the player to read. Sometimes, if there's too much text, a player could be disinterested and try to skim a bit to try and get to the game portion of your story. Which I think would suck because your story writing is really neat and very original. The song fit as well and it didn't feel overwhelming or unwelcoming at all. Very VERY nice job on this one!