From my understanding of the story, it's about the cycle of abuse. You see how the parents were in an unhealthy relationship, where the father only cared about his wife and the mother remained by her husband's side despite the abuse. The father hated sharing the mother, but the mother of course loved her children, and in the end, the father's anger and jealousy killed his wife. While the two boys were taken in by relatives, they were left marked by their pasts. The older brother wants to have some sort of control and abuses the younger brother, ensuring that the younger brother becomes dependent on the older brother and thus never leaves him behind. The Younger Brother due to the trauma and love of his family accepts the abuse because he sees it as part of being loved and loving someone back. Both are wrong in the head because of their past and no one is actually helping them either, making their situation worse.
Of course many ppl saw this story as glorifying abuse and incest (thus the backlash). That is why the author gave up on it. From what I remember reading, the ending included the older brother going to jail or dying. One of those two, but the younger brother actually begins to heal once he is away from his brother. Ultimately it was supposed to show that the codependent relationship the brothers had needed to end for both of them to move on and break the cycle of abuse.
From my recollection of her blog (and probably AWFUL Google lost in translation) my impression was that the mangaka saw people liking the series and was just "No, this is meant to be an awful situation, is that how people are taking from my work as?" and didn't want to continue further because it's feeding into fantasies.
Again, I don't know how much was lost in translation: but from what I saw of her blog, she also raised in a bad family environment, and (my words here) [so she wrote Camellia as a way to cope/reflect/revisit], but then readers didn't take the message she wanted out of it. It's unfortunate.

OK, so what I got from the note in the extras from the person who uploaded this is that the author got frustrated that people were missing the message of this manga now tell me what message what message is supposed to be evoked.
I’m cringing ,also does anyone know what happens in the novel cause I know she deleted everything but like if somebody does remember or something please let us know.