I'm a librarian who loves anime, manga, and reading a wide variety of genres.
(The pages aren't numbered, so I'm guessing.)
[Souji, after waking up naked next to Saki.] "This is how it is between Saki and me. It's been this way for awhile. Saki is the daughter of my father and his mistress. It almost seems destined that we would end up like this, just like our parents. And not just me, but probably Kazushi-niisan as well."
Eww. No. For one thing, presumably Souji's father and his mistress weren't related by blood. For another, no, it wasn't destined.
Don't have sex with your half-sister. And your brother shouldn't be having sex with her either.
ETA: Also, I'm pretty sure the translator meant "a while" and not "awhile."