I don't remember having a brother on itch.io, but still :-)
I just read the plot Deimus wrote in dialogue and use hints from characters in the game. And in the library.
It's right there in the name: hidden quest. It means that you have to go out and find them. Just keep interacting with everything and everybody you can and eventually you'll get them. (Yes, it can be frustrating at times. Join the club.)
This forum is also a good place to look for hints. Scroll down and read what others have had to say about it.