Why is Ilsa Hermann ‘insane’?

I don't think she is insane at all. An intelligent woman with her own library, Ilsa has spent decades mourning the death of her son Johann, who froze to death in 1918, the final year of World War I. Ilsa takes a liking to Liesel: she witnesses Liesel stealing a book from the book burning and invites Liesel into her home library to read. Ilsa gives Liesel a blank book and encourages her to write, and not to live the rest of her life in despair. Ilsa and her husband take Liesel into their home after Hans and Rosa are killed.