nono, its more like this I believe
- both the sister and brother can transmigrate into a novel, and also out I believe (I dont really know fully why the brother is stronger??? but he just is)
- the siblings also hate each other (shocker)
- the brother (I think) is obsessed with our mc, to the point where he really wants to kinda just lock up our mc away. thus, this is why he decided to trap him in the novel, as the mc just so happened to pick it up while in the brother's room
- I think, if I remember correctly, the sister can place people into books, while the brother can place people in and take them out. thus, why our mc was able to go both ways with yoon present, but only able to go in when the sister (yoona?? I forget her name...) was present
most importantly really, its that the brother is possessive over the mc, thus locking him into the bl novel. I dont think it was the sister's doing (putting him into the novel), but at that exact time of forcing the mc into the book, I guess yoon (the brother) had a really posessive streak or arc?? im not absolutely certain, so take what I say with a grain of salt.
I feel like the powers of the siblings wasn't given an explanation at all, or their relationship for that matter. I mean, that's how it all started right? The sister hating her brother, hence taking away the person he loved most into a bl novel? There has to be something more!