As a mental  health professional I would be very very very cautious about
making any changes (or suggesting any changes) to your sister regarding her
treatment plan.

If she has been stabilized on medications I would be very grateful for that
not small victory.

Any dietary changes should probably be approached for the purpose of
improving overall physical health.  (Most folks on neuroleptic medication
are somewhat overweight).  Before broaching a diet change talk with her
Treatment team.

HTH
Pete