My 85 year old mother is the sole guardian of my 60 year old sister. What would happen if my mother passess or is unable to be guardian?

If your mother dies, a new guardian has to be appointed for your sister.  The procedure is not as difficult, because the first time your sister had to be found to be incompetent, and then a guardian appointed.  This time, all that will be necessary is the appointment of a substitute guardian.  You can do it now, appoint a co-guadian, so that if something happens to your mother someone is already available.