I believe this manga hides it's true beauty behind a thrashy art style and a silly story.
He is unhappy with how his stepsister is treating him with disgust, so he wrote a light novel with step-siblings loving each other. Although he probably went overboard with the love aspect because judging from the few lines from the novel, the siblings from the novel have a lewd relationship.
Unintentionally, she became a huge fan of the light novel, even going so far calling it her holy scripture.
After reading the novel, she clearly liked the dynamic of step-siblings loving one another very much, and wanted the both of them to have the exact same relationship, as evident by her trying to reenact the scenarios in the novel irl. Unfortunately, she can't easily have that kind of relationship with his brother that easily, as they don't have the same relationship as the siblings in the light novel, in fact, they have the exact opposite relationship.
So what does she do? The only thing that can make her brother love her like the one in the novel, hypnotism.
He is unhappy that his sister tried to hypnotize him and he is forced to do these lewd stuff, when in fact, if we really look, he is the reason why she did it in the first place. The author managed to sneak in this interesting butterfly effect loop pretty well.
People have been calling this manga trash. But there is a popular saying: "One man's trash is another man's treasure". This manga might look like trash, but I believe if we just give it a closer look, it will reveal itself to be, in fact, a treasure.