I don't know about this one, chief. Kurumi I can deal with because IIRC there's only supposed to be a 1 year gap between them. But with this girl, even if you ignore the whole incest thing which is in and of itself hard to ignore, she's 12/13 while Rentarou is 15/16. Even in the best case scenario, a two year gap at the age they're at is fucking huge in regards to maturity. Worst case is that she's 12 and he's 16 and I shouldn't have to speak any further on why that's sus.
I can't see this addition as anything other than a colossal Oof.