I'm jumping in a little late just to relate a personal experience, not a wedding, but our daughter's high school graduation a year ago.
Now, my ex and his paramour, the OW, have long since parted ways, so the woman he brought to our daughter's graduation is someone who came into the picture long after our divorce. I was a bit perturbed, as I struggled to understand why he was bringing her. She had always been referenced as his assistant. I knew she lived in the same apartment, but again, it was always framed as a money-saving situation. OK, naive-me, I let all those explanations wash over me and accepted them at face value. Mainly, because I didn't care, anymore.
Come the graduation, and they show up, horribly late and also inappropriately dressed. I took our party out to dinner, and my ex couldn't sit still...was up every ten minute to go out and have a smoke. Later, back at my home, he never really came in, just waited outside while his "assistant" sat and talked with us and had dessert. She really isn't a "bad" person, but she's a little strange...multiple visible body piercings that seem out of place on a woman her age.
I ended up driving them home that night as the bus trip would have taken a couple of hours what with connections in the city. I got them home and was back to my house within that time. The graduating daughter accompanied us for the trip, and on the way home when it was just the two of us, I asked her, "Just what is your father's relationship with D?"
"They're friends with benefits."
"OK. Why the big deal trying to make it seem less than it was?"
"Well, we knew how upset you got when Daddy was with E [the OW]. I just wanted to make things easier."
"Honey, I could care less what your Daddy does these days...or with whom, as long as it doesn't hurt you. Understand?"
OK, so it was my ex who had a partner with him for the event, not me. Yet, he was much more uncomfortable than I was. Maybe because I took control of what I could control. I planned the dinner; I hosted the after-dinner party; I graciously drove them home.
Now, bacall, you might not be able to do as much as I did, but you certainly could, as mother-of-the-groom take the reigns for planning the rehearsal dinner and have some measure of control. It helps with confidence, and it helps solidify your position. I'm not suggesting you go out of your way to make your ex and his new wife uncomfortable, but you need to find something that will give you an equal measure of comfort that you seem to ascribe to them being a couple.
And you have lots of prayers from me that you find the way.