I just wanted to say that we had the same problem with my mother, and if you do a search here on TP, you will see many others have as well.
We had the same pattern as your mum, with repetitive phone calls, mainly in the evenings. It got to the point where I would cry every time the phone rang.
My advice for your sister is going to sound unkind, and I want you to know I do not mean to be unkind! I understand that your sister is, understandably, concerned about your mother. We all know that feeling of dreading the "emergency" phone call. But it really comes down to this: your sister does not have to answer the phone, but can choose to answer the phone every time it rings, or not. I recommend, for her sanity, not answering more than once an evening. If the phone calls come after your sister's bedtime, she can turn off the ringer.
The separate phone lines, with a dedicated message, are a good solution for some people, as is blocking certain phone numbers. My mother's memory is to the point where she cannot remember phone numbers, and we have given her (many copies) a phone list with just the numbers she needs.
Best of luck!