Meeting my on/off girl’s ex-fwb

We’re both in our 30s, she’s older(let’s call her D) and used to work at a science institute that has a college for people doing their Ph.D. We’re strong together and everything, like soulmate-shit strong. We can’t be together but we keep coming back to each other over the last 7 or so years. I’ve had a non-existent dating life, whereas she’s been quite social, with relationships, random hookups, etc.

During one of our off times 3 or 4 years ago, she became good friends with one of these students at the institute. They’d hang around the institute when she didn’t have much work. I don’t know much about how they got close, but it got to a point where he proposed that she allow him to masturbate to her feet and she obliged. The only detail I know is that this wasn’t about sharing feet pics, but her being physically present in his dorm room and using her feet to bring him to climax. Quite a few times, from what D told me. They stopped after he found a girl that he’s still dating to this day and I’m told they’re going strong, but D and the guy remain good friends.
D and I have been on for a good part of the last 3 years. D told me all about it a couple of years ago and I didn’t take it too well for a bit but then I was okay since we were barely talking at the time. Cut to yesterday, when we make plans to hang out at her place for the weekend, and then she tells me that she’s expecting guests. Turns out the dude from the institute and his girl are coming over too.
Now, given my history, I’ve never been in a position like this and I find myself anxious and troubled at what am I to think of this or how to approach this situation. I’ve never been in the same room as the woman I love and his ex-friend with benefits. My mind keeps going to a place where I feel like I’m secondary or something, like I cant really put it in words. Is this a need-to-be-alpha instinct or something? How do I conduct myself in a situation like this?