@UnknownSaint111 Older brother thinks his sister is making a shoujo manga so he tries to help her by modeling for her and doing shoujo manga actions but she's actually making a BL manga. It's kinda funny.
The title is slightly incorrect. The proper translation would be "BL manga brain sister x Shoujo manga brain brother" as in the sister has BL on her mind and the brother has shoujo mangas in his mind. The title makes it seem like the brother will be writing shoujo manga too.