She was never in good health, as she was immunocompromised from an early age. Crohn's Disease or intestinal tuberculosis was what she died from, I understand, but she was wracked with all sorts of stuff.

If you want to get into the story of Lili in WWI, you might find this 2012 music & gender studies thesis on her interesting: https://depot.library.wisc.edu/repository/fedora/1711.dl:W6F... .