I Have A Sister Who Is In Her Mid 40's

I have a sister who is in her mid 40's who can not stop talking for hours, switching from subject to subject then repeating the same stories 3 or 4 times in the same conversation almost word for word. For the longest time my thought was that she is self centered/selfish/narcissistic; especially when she interrupts me when I finally speak to talk to her cat or start a new conversation about herself again. Then someone mentioned that maybe it could be a form of compulsive behavior that she doesn't realize. Is that a possibility? She was always a bit like this, but the last 2 years there has been a significant increase; I even avoid spending time with her and don't answer the phone because it drives me a bit crazy.