I think how appropriate talking about others is depends on why you are doing it. If you are gossipping, that is a waste of time at best and could be much worse if it is negative and or untrue gossip.
On the other hand, you may talk about your interactions with others just because you are talking about your own life and this other person is a major factor in your life.
You could also talk about someone in a directly productive way, such as discussing candidates for a job or asking for advice or assistance on behalf of someone else.
It sounds like your sister's conversations are towards the gossip end of the spectrum, but it is important to note that not all discussion of others is like that.
On the other hand, you may talk about your interactions with others just because you are talking about your own life and this other person is a major factor in your life.
You could also talk about someone in a directly productive way, such as discussing candidates for a job or asking for advice or assistance on behalf of someone else.
It sounds like your sister's conversations are towards the gossip end of the spectrum, but it is important to note that not all discussion of others is like that.