Thank you for this question about phone courtesy.
In fact this is considered rude in most business circles. The only acceptable way to take a call such as this, in terms of courtesies, is to ask permission, and then only if it is an important expected phone call.
In my business circles, we inform persons up front, that we have time to talk, but that I may be interrupted by an important income call; then ask "is it ok if I take that call momentarily". In most cases we get told it is ok.
In so doing, it is understood, that we do not have several such important calls.
it is expected that if we have agreed to talk no the phone, that those obstacles are cleared right away, or that we respect the person by not taking the call. The caller can always leave a message if your sister does not pick up. And she can return the call later when you are done.
However, such courtesies are forgotten when people are in personal circumstances.
It is rude and inconsiderate. When she is in communication with you, you deserve each other's undivided attention. It is only respectful.
In my circles, we do not even have a cell phone turned on during dinner, in private clubs, or on dates. If we need it turned on during a date because of that important call we expect, we clear it with each other, and do not answer the phone unless that one call comes in.
What your sister is doing is considered inconsiderate and rude. It is not being respectful of you and your needs.