So I went back and checked, and I believe Sister has read only till VII which titled the fallen angel's cage. Seeing as how one of her wishes is to read the rest of paradise lost, she's probably not reached XI which is titled a seductive night and that's the one with the Yuri.
I mean, after reading ch 21, done by spining potato, it seems that everyone is sister-sexual but sister herself is god-sexual.
Also, kinda cultured of the author to use paradise lost, never thought I'd see that in a manga.