They have been together around 10 years and have been open the whole time. They are both in platonic marriages with other people. SO and I have known each other forever but just recently decided to take it to another level I feel like yelling at her because it irritates me and I start feeling like I did when I was a child and my sister would copy me. It's childish but that is how I feel. However, the grown up part of me knows that is not the answer.
Personally I think it bothers me more because I'm not fond of her. I tried at the beginning to be closer but she drives me crazy so now I tolerate her. She is a bit unstable and has a severe mental health issues that have rocked my relationship with SO. Her mood swings and meltdowns makes me walk on egg shells. However, my SO cares about her so I do my best.