You and your family and/or friends can be her audience. Since you guys are close to her, she'll feel more comfortable. Encourage her by giving her compliments and push her forward. Pretty soon, she'll be just as outgoing as you! I wish your sister best of luck! :)
Have her practice in front of you then friends then family then smaller crowds that get bigger time after time and make her feel confident and if she is self conscious then compliment her and make her feel like the world wants to hear her voice. Hope it helps! :)
well I had stage fright but to overcome my fear I went on stage and was about 2,000 people watching me and everyone else sing so just talk her in to it . If she going to be a singer and dancer she has to over come her fear.