My sister has anxiety disorders. What should I tell her?

Psychotherapy. Suggest to her that she seek a competent psychotherapist. An anxiety disorder is a fear of a hidden object. A therapist can help her learn what the hidden fear is and deal with it. An anxiety disorder is no fun. Anxiety can also be a presenting symptom of depression.
Get Expert Help. Get talk therapy that goes well beyond the line, "tell me how you feel about that." first, u really do need someone w/training & expertise in treatment of anxiety problems/disorders. It's vital 2 understand what anxiety is, how it ticks & what specific things can help if anxiety starts interfering w/ ability 2function & w ability 2 form/keep relationships. Great info here: http://www.Anxietybc.Org.
Relaxation exercises. Exercise daily, mediterranean diet. Relaxation response by h benson md (harvard prof) excellent book still in print after 25 year http://www.Webmd.Com/balance/stress-management/features/blissing-out-10-relaxation-techniques-reduce-stress-spot?Page=3.