I am 27 years old and when I was in high school I had a relationship with a guy my age with whom I had fallen deeply in love. We went out for about six months without consummating our relationship and when we broke up I had a hard time getting over it. Of course, he was the one who left me, without explaining why. A year ago, I realized that he had a relationship with my younger sister who is 22 years old. I asked her about him, if they were together and so on, and she told me that he had talked to her about the relationship we had in the past. She told me that she was in love with him and that she went out with him because she did not imagine that it would bother me since I had become engaged. And I told her I didn't care, she could do whatever she wanted. At that moment I believed it. In the process, however, their relationship developed, he proposed to her and now I see him constantly in front of me with her, and all this creates strange feelings in me. There are times when I feel angry and jealous of my sister. There are times when I don't want to see them together. I try to hide it from my fiancé, I try to avoid meeting them, but it's not very easy, because we live in a very small town, and besides, my sister and I still live with our parents. I can't understand why I feel this way, and I don't know how to handle it. What should I do?
It's really awkward to see your sister with a man you fell in love with at some point in your life, even if the relationship had not progressed. It is a constant irritation that takes you to the past, especially since your relationship with this person has left you with some unanswered questions, such as why he left. Perhaps the practical solution to this would be to speed up your escape from your parents’ house, perhaps by moving in with your significant other a little earlier.
However, what you need to think about is why you feel the specific feelings towards your sister. You write to me that you feel anger and jealousy. You're angry with your sister, but why do you feel betrayed by her having a relationship with your ex? If this is true, how do you interpret it given you have already settled down with another man, and from the moment you told her that having this relationship does not bother you? Does this anger have its roots in your relationship with yourself? And if so, why are you angry with yourself?
Similarly, what is often behind jealousy is a sense of self-doubt about a feeling of inferiority to the one we are jealous of. Jealousy, therefore, has less to do with the person we envy than with ourselves and the image we have of ourselves. So why do you feel that you are lagging behind your sister? In which part do you not feel good enough? Why do you feel that way?
I would also suggest you reconsider your relationship with your fiancé and whether this relationship fulfills you and satisfies you. Is there anything in this relationship that you find difficult to accept, situations that do not satisfy you, that make you insecure about the future? Does this lead to comparisons with your sister's relationship and that's why you can't stand to see them together?
I would advise you to seek the help of a specialist. By talking to him you will be able to better understand yourself and your reactions.