Am I overreacting by being paranoid/upset that my boyfriends ex is dating his twin brother?

To summarize this mess in my head, I have been in a relationship with my boyfriend for almost 7 years. My boyfriend's identical twin is dating my boyfriend's ex. I initially thought they only kissed, but later found out they were together for 4 months and were intimate. This was 12-13 years ago, however, I feel extremely uncomfortable and disturbed by this situation, experiencing nightmares and feeling sick whenever I am around her. Even more so because when I am around her, she constantly talks about her relationship and her ex (not my boyfriend) when we are alone together, making me even more uncomfortable. Despite expressing my feelings to her, she continues her behavior, which is what is leading me to seek advice and feedback on how to move past this situation.

Am I overreacting by being paranoid/upset that my boyfriends ex is dating his twin brother?
Post Opinion