What's with all the hate with Botan?
No doubt that she's in the wrong being in a sexual relationship with a married man, but the thing can be resolved by settling it using the adult matter. The relationship could be kept in the dark if Momiji didn't just throw all the facts to the family.
Look at what she done for the family;
When Kashiwa suddenly come back, and said she wanted to be a female. At first, she was in total confusion, but she did not deny the feeling that Kashiwa wanted to be a female. After that she went and check the cost of getting a sex change while the two little sisters are still not accepting the fact that Kashiwa wanted to be a female.
She did not change how her younger sister's attitude towards hobbies and love life.
She gave up her education and come out to the working society just to aid the family in their future education.
What about the married man and Botan?
That guy told her about his wife made him the bento. Botan even told him to not throw away the feelings his wife had (in the bento) since the family love is free and endless.
To be honest, their relationship is wrong, but who is the one to make things going to the wrong route are the adults, not the innocent child that the man have.
Never ever put an innocent child in front of this matter, no one likes it. Adult got to do what adult do.
Botan's family told her to find someone who she can marry and not find someone that she loves. She's fine being the one left out as long as the relationship is kept properly instead of the situation like this. When she was 18, she need to do what a normal 18 years old does not need to do. She need to raise this family selflessly, no matter what she want, she NEED to put the family first before anything. Yet, she not allow to have this feelings towards a man just because he's married. She's wrong having this feelings, but this is the only time she is thinking for herself. Yet the family just want to think what's just and what's wrong.
She's basically tired of this world that she need to think for the family instead of herself, that's why she left. Isn't it good? I mean, since the family don't understand how she feel, don't you think it's time to take one step back and think of the whole situation instead of throwing the whole blame towards Botan and the married man?
In summary, no one's absolutely right, no one's absolutely wrong either. The only thing to blame is that the adult world is not something that can be easily understand.