My sister says she has addison's disease. are family members more likely to get the same thing?

Addison's disease: Addison's disease (below normal function of the adrenal gland) is an autoimmune disease. If one member of a family has autoimmune disease, then other members may too (risk about 1-2%). But autoimmune disease can effect any organ, most common the thyroid, but also liver, adrenal, red blood cells, ovaries, lung, etc. Thus it is not possible to predict what kind of symptoms others might get.

Higher chances: In modern times, addison's is primarily an autoimmune condition and therefore, as in all such conditions, there is a higher incidence of same condition in immediate family.