You over thinking the situation from my point of view.  He was honest with you, you mentioned no sign of him trying to get with her or vice versa. The incident you write about is getting the best of you , you say you forgave him then do so as well as her . Her being around and your jealousy will only cause you grief and your relationship will suffer .

Dear Soulonfire,

Sometimes we create our own demons. These bad things are figmentation of our imagination and we give them so much time and start acting in defense to them that they become real. I have my personal experience in this. Don’t overthink things.

If you want to overthink…. think about your partner, of his well being, ways you can make his life better. You are just scared that someone you loved will fade away in other relationship. Don’t give him reason to fade away. Love him more, care for him more. Get out with him more share your adventures more and make the other girl jealous. If you are then stop discussing her with your boyfriend altogether. Don’t feed your demons to your partner.

Stop treating the other girl as your boyfriend’s ex and treat her as his sister’s friend. If you are living in past, you will eventually let her spoil your future. She is no one just a friend of his sister and nothing more.

If your partner is mentioning her more often, grab this opportunity. Show him that you are a bit jealous, love him more by making his next moments special. He will like it and love you more (I know I will).

Best of luck.