I am so sorry you had a falling out with your sister. I have a similar situation with my younger sister too. I am the one who gets cancelled on and I always try to overlook it. I told her nicely a few months ago that it hurt me when she cancelled so much. Since then, I have not made many plans with her. I would just give your sister some space and let her issues work themselves out. We are here for you when you need us. I did this with my sister and we are getting along much better. I hope this works for you too.