I think a good test to judge a manga, specially with sus aspects, is to ask why?. Why would the girl they adopt is very childish?, welp, she was obviously neglected by her parents and therefore did not have a common upbringing. Why is does this girl have such a developed body? Well, you could argue that she is just puberty and a lot of women pass through the same body changes. Why would she fall in love with her adoptive brother? In this case it is not actual incest and affection was bound to happen.
Up to this point, you can dodge the previous questions with simplistic answers, but then we go into real questions. Why is a family that care so much for an adoptive girl not teach her how to use a bra, and then they put that responsibility under the brother who is also the boyfriend? Why until 50 episodes in they show you that she actually studies, only as an excuse to show her in a school uniform? Why is the family weirdly ok with the "incestuous" relationship to the point they encourage it?. Why make (not reveal, its an issue of the author, not story) the girl 14? Then the answer is even more simple, it's a fetish, the whole manga is a glorified fetish fantasy of "you adopt an innocent child who ends up falling in love with you".
For me, that just overrides the "wholesome" parts of the story. Then it becomes even worse with the dramatic elements that introduce the mother as the villain, implying that the main characters are the "righteous" ones and in charge of redeeming her.