Even during the first dinner, there is a bunch of inconsistencies though.Incest is a minor aspect currently, but the premise and interaction with those characters only really makes sense with the incest patch and pretty sure the patch is official.
"And it will be more fun for my girls now that they have a housemate. I hope you can become friends."
"Cynthia is a year younger than you. You'll go to college together."
"Nice, I have an older brother now! And we're going to the same college!"
So I can think of 3 possibilities...
Incest patch is fan made jank.
Incest patch is dev made jank.
The mom divorced MC's dad right after MC was born and the 2 sisters are half sisters.
Hmmm... I think i'll play without the patch. If it turns out non-incest also doesn't make sense, this is one of those games that can't decide if it wants to be incest or not.