This is one reason why I absolutely hate mobile phones, it has made it impossible to have uninterrupted time alone, unless you are strong willed enough to switch it off. In the days before we had them, people would ring your landline, and if you didn't answer would leave a message (or not), their choice, and you could choose (or not) to get back to them. I therefore think that 'SunnySusie' has the right idea, tell your sister there are certain times when you're busy and can't chat, but you'll give her a call at least once a day to make sure she's alright. If you have a landline, put the answerphone on if you have one, and if you really must have a mobile switched on at all times, put it on answer phone setting too, then just choose to take some of her calls.
I realise that this may seem harsh, but if you get so fed up that you end up blowing your top with her, it will upset her far more than not being able to call you 20 times a day.