Publishing language: Unknown.
I read this book in school many years ago, then in my native language: Swedish. Likely it was originally in some other language.
What I remember:
The protagonist is writing a story where the main character (his alter ego) talks to him directly. Later the boy's sister writes, which results in her alter ego appearing in the story. It is set in "modern times", since I remember the boy writing parts of the story using a modern day computer.
The book had illustrations but I do not remember it being a graphic novel.