Thanks for coming to HCM. Sorry to hear about your sister's condition. Langerhans' Cell Histiocytosis
is a rare disease and treatment depends a lot on extent of systemic involvement and localization of disease. I am sure her doctors will work out best treatment regimen for her.
You can help her by being supportive during this difficult phase & the course of her treatment, try to keep her positive and hopeful about recovery all the time. These days, most hospitals have good supportive care teams who help the patient & family emotionally strong during this difficult time.
Oncologists from HCM can comment further.
I wish her a speedy recovery!