Answer:
my mother has really bad anxiety, and its also really hard on my father and i. the main thing you do is be blunt with her, and tell her whats wrong. but you must also be considerate with her since she is going through a tough time. something that helps is distracting yourself (or in your case your sister) from her thoughts and her fear. do stuff with her and dont idolize her anxiety, and after a while it will become better. dont let her feel alone in this journey of life. it either worsens or gets better.
please, if you or your sister needs to call someone, i would recommend the anxiety helpline, or su,icide prevention hotline.
ah - 1-800-950-6264
sph - 1-800-273-8255