I really think both the author and editor forgot about FMC being a first year college student because they haven't mentioned\shown her going to class and now you have Mahiru staying over forgetting that she is a highschooler at the eldest so what about her schooling. (Totally forgot this but in order for Mahiru to get married she would need permission from her guardian since ahe is underage so how did she get it from her big sister when their parents died)
Pages 7&8
If BB Kyousuke is like that, that would mean he asked Miharu and she agreed to kick her out to stay with a person they never met. If Mahiru always thought about FMC she would check MC out before agreeing to it. She would constantly be checking in on FMC but she didn't
Page 21
Why did she send FMC off in the first place if she has that much of a sis-con tendency?