lul yall read the same chapter, right? Her character: Firm, proper, clean. She was pressured by peers and adults alike, albeit unintentionally probably, to be this sort of proper girl and from all the manga and anime we have seen, a slice of life genre orientated, these designs for the characters are not supposed to stand out. A prim and proper student just starting junior high, probably just straight out of elementary school, a time in every human's life where our environment and people around us influence us, is being changed here due to someone finally speaking her feelings without having to be told how she feels. If there's one thing I'm sure everyone can understand, is that high expectations for a child can change them, just as it changed Chiyo here.
And as for the part about incest, literally, we have dealt with a mother who lusts after her own daughter and her daughter's boyfriend. She is also in a 13-way relationship with not only other girls but also her daughter and her daughter's bf. Now how the mangaka is gonna traverse this very thin line, idfk but we have already accepted a relationship involving multiple people ( polyamorous relationship), a boyfriend they share, a gluttonous middle schooler who has been here for a while already, and an adult who not only lusts after her daughter's bf but also her own daughter as well as being the principal or chairwoman of the school they all go to.
Sooo kinda too late for us, or at least those who are trying to put down or disapprove of this new girl, to try and play the moral high ground here. Especially since we know there is gonna be 100 girls that will have their own significant personalities as well as physical traits that make them stand out. Rentarou genuinely loves all of his girlfriends (or will be written that way) and wants to not only save them but make sure they live happy lives as long as he is around.