At page 40,41 Kenji gives a handjob/frottage to Kazuma so it's R18 as the covers shows. The author simply didn't draw the penis because of the censorship in Japan, something what a lot of authors did in older books, which i always really hated it it's just lazy and kills the erotic parts of the books.
I can't understand it but i hope Kenji didn't just fooled around with Kazuma meanwhile he gave birth to his sister. That's a huuuuuuge turn off.