The patreon site describes the MC as a "university student"(pretty clearly false, he's obviously in high school). His sister/roommate Sara is in a different class than he is and appears to be younger than the MC(who himself is more likely to be 17 than 18).
If you change the descriptors back from familial terms, the inherent tension/conflict makes absolutely no sense. "We can't do this, I'm your [landlord/longtime family friend/roommate/too old for you]" doesn't come close to "we're blood relatives".
Technically, you are correct, none of these are explicit. It is, however, pretty implicit that they're blood relatives and that some characters are under the age of 18.
Plus, y'know, the dev had two previous games called Incest Story 1 and Incest Story 2, and his "handle" is ICSTOR. I'm just chuckling at the ludicrousness of the workaround necessary to stay on Patreon.