Congratulations on a job well done. Even far from that - don't remember the last time I've been amazed and thrilled by that kind of games. And I have to admit that I didn't like horror types. Still you got me with this. Drink a beer/or cocktail from ME. If I can I could donate some amounts $$ in future.
Now MY RECAP: (1-2 para)
It seems that as I get it–her brother from the start also did some research and did try a lot of rituals, but it seems he got stuck. That is way he didn't or can't come with her. So he remain. No spoilers but the person named Sai isn't quite him (knew it from start :). 
Cheers from me ૮ ˶ᵔ ᵕ ᵔ˶ ა and stay safe ᕦ(ò_ó)ᕤ