I read your query and I am sorry for your sister's illness.
Regarding the question about the cortisone cream, the answer is no, that has definitely not been the cause of her having multiple sclerosis. Actually
corticosteroids are used at times as treatment during the course of the disease.
As for the treatment there are short term treatments when the patient has a relapse period when corticosteroids are used for some days. There is also long term treatment aiming at slowing the progression of the disease and reducing frequency of relapses. The
interferon group of drugs are the drugs traditionally used for that, but in recent years there are several promising new drugs whose long term efficacy has to be seen. They are all very expensive though both the interferons and the new drugs, so at times getting them is a question of where you live and their reimbursement by your country.
Of course for specific symptoms (
spasticity,
depression, urinary disturbances or infections) other drugs may be added to address those symptoms (but not influencing the course of the disease) as well as
physical therapy.
Hope to have been of help, feel free to ask further questions.