My boyfriend?s sister brings up his ex in every conversation.

TL;DR my boyfriend?s sister manages to bring up his ex girlfriend after us being 5 years together.
So my boyfriend was with his ex for four years until she cheated on him. They broke up and afterwards, he started dating me. We?ve been together for five years now. His sister and I started talking more over quarantine and I thought we were getting closer as friends. However, I started noticing she always manages to bring up my boyfriend?s ex girlfriend in our conversation. She?ll bring it up by saying,? I don?t mean to bring her up, but..? and then proceed to talk about my boyfriend and his relationship with his ex. I don?t know how to handle this situation. 1 because the way she brings it up is so casual I feel like it should not be bothering me, and 2, should it even bother me? I don?t understand why it is still relevant if they broke up more than 5 years ago, she cheated on him, and they didn?t have any kids. How can I go about dealing with this?
Also, I should add that she only brings it up to me and never when it is my boyfriend and I together. She asks questions about their relationship and makes comments about it only to me. I?ve told my boyfriend about it and he tells me to either bring it up and tell her to stop or just stop talking to her if she?s always bringing it up. What could be the best way to handle this?


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