<human>: My good friend who I admit I hve a passive aggressive rivalry with is going to be here around Christmas for a week. I want to see her, I'm excited to see her. 

She and my current SO used to have sex occasionally when they happened to run into eachother. They haven't in a couple years. I know she had feelings for him at one point. 

I know there's going to be a lot of opportunity for us to all be together and I am so uncomfortable with it. He has never given me any reason not to trust him and I don't have a history of being cheated on. But something is just rubbing me the wrong way. The ooonly thing I can say is one time he told me he might have dated her if timing was right (all of us have moved away from our hometown once or more). I've never been in a situation of having love interests in common with my friends.

How do I give myself more confidence about this situation and give both my SO and friend the trust they deserve?

Condense the text into a few key points
<bot>: My SO and friend, who used to have sex, are going to be in social situations together (with me) over the holidays while she's visiting. I'm super uncomfortable with it.