I am sorry to hear about your sister's diagnosis but know that it is good that she knows and can be treated. Having Addison's disease is a lifetime commitment. She will be on steriods for the rest of her life to replace the cortisol her adrenal glands do not make. If her adrenal glands are working at all it is a very minimal amount and not enough to keep her alive for a long time. You are right that as time goes on her adrenal will produce no cortisol. There are some alternative treatments for mild adrenal fatigue but if she was diagnosed with Addison's disease the chances are very good that she does not have a mild case of adrenal fatigue but a severe case.
The best way you can help her is to be supportive of her challenges which can include; time of crisis from a simple cold or from increased stress where she may need very high levels of steriod called stress doses to keep her from going into an Addisonian crisis which usually ends up in going to the hospital in crisis. There are many websites which can give you a lot of information on Addison's and the usual treatments. I am on hydrocortisone 20mg/day(10mg. at 7am, 5mg at noon and 5 mg at 3pm) and .1mg of fludrocortisone. This is a typical daily med schedule for AD. Some people are on predisone instead of hydrocortisone. It is also very important for your sister to get a medical alert bracelet to let medical staff know her disease in case of emergency. I got mine from n-styleid.com, they have really nice bracelets at a very affordable price. I had the back engraved with the following: adrenal insufficiency steriod dependent. I also have a hydrocortisone injector in case of emergency and anti-nausea medication which really helps if needed.
Good luck with your search for your sister; she is very lucky to have you in her corner!