Problem with partner's lovers
My ex husband was sleeping with my girlfriend and things were tense. She wanted me to be there for her and share all about her time with my ex. I wanted him back in loving relationship with me and he wanted her without me in the picture so it was convoluted. It did not end well. None of us saw each other after the big blow up. However, she and I have spoken kindly as friends and he and I have met again amicably recently. He asked if I wanted to be with him but at this point I have a working triad and was no longer interested. We are all better off though as friends than lovers at the time not one of us was clear about communication or even what we needed and wanted from the relationship.
My current partners and I work really hard on clear and focused communication. Everyone has a right to be heard. We all lay what we want on the table, sometimes we get it and sometimes not but at least we are all clear on what is needed. It works much better.
I hope that you will take the time to visit with your sis and really focus on peace and happiness and not let your guilt, jealousy or doubt interfere.