You can use 'onii chan' as a term of endearment or a nickname for a character. Maybe have a character call their older brother that to show a close and affectionate relationship.
Well, 'onii chan' is often used to refer to an older brother figure in Japanese media. In a light novel, if 'onii chan is done for', it might imply that his story arc has come to a negative conclusion. For example, he could have lost his powers if he was a supernatural character, or his reputation could be completely ruined in the social hierarchy of the story world.
It could be a complex story involving various emotions and circumstances. Maybe it's about an unusual relationship and the challenges that come with it.
Not really. It's a rather specific statement. While the concept of a character being 'done for' can occur in light novels, this exact phrase isn't that common. Most often, fans will use more detailed descriptions like 'the character's arc ended badly' or 'he's in a hopeless situation now' rather than this specific wording.
I recommend a novel called "Detective Conan: Onii-chan's Resurrection Plan" to you! This book was a light novel written by Youzi Shu, a derivative doujinshi novel. In this novel, the protagonist was transported to the world of Detective Conan and regained the opportunity to become Xiao Lan. Onii-chan skillfully used her memory and detective skills to solve many complicated cases with Conan. I hope you like my recommendation. Muah ~