Have you asked her why she feels that way?
TMT, D22 has always been scared of gushy overt emotions. I think her fear is that I would be emotional and that she would then also feel emotional.
It's all a moot point anyway... because I went to the party, xH and OW showed up after a few hours... and it was no big deal.
I wasn't bothered at all (and in fact felt a bit sorry for the both of them; they were fairly obviously feeling more unsure in the situation than I was).
When they arrived we were out on the back deck and I was shielded from them coming out the back door by M and one of D24... sorry D25s! friends. D22 and extra-kid D23 were sitting facing us and the door. They got all 'they're here, they're here' whispery to me and D22 looked a little uncomfortable. D23 was being funny and making sly 'upper cut' motions at me and winking to diffuse the tension, haha. xH was greeted everyone and then came straight over to M and introduced himself and OW (with a firm handshake). Then came around M and gave me his standard hug and kiss on the head (I'm short!) and said 'thanks pet-name-for-Ever' with great sincerity.
We all got back to mingling (it was easy and no big deal to steer clear of OW, enough other people to chat to) until it was cake time. After D25 blew out her candles and we'd (badly) sung Happy Birthday, xH made a brief speech where he said something like 'it's been a bit of a $h!te time the past few years' (or something like that, it was M that picked up that he'd said that and told me after that he found it interesting). Then he went on to include me in the speech and talk about 'we' and 'our daughter'. Honestly, if you didn't know we weren't together and you heard the speech you'd think we still were.
Of course he did this also at D25's 20th party, which was only a couple of months after he had BD'd me (no OW there at that one), so I wasn't really surprised. He's consistently wanted to feel like we are still a 'family' (keeping the family health insurance is another way he does this). Reflecting, if I was OW I would have felt pretty uncomfortable in that scenario. But I'm not OW and I felt completely at ease. So weird after all my fear of what it would feel like.
M was a legend. There when I needed him, mingling and talking to the girls' friends. THEN, towards the end of the evening (well for us, we left about 9.30), I was inside talking with the three girls and D23's BF... and we look out onto the deck and see xH go up to M and start chatting. They chatted for at least half an hour. We were all laughing (inconspicuously of course) at how strange it all was!
When I felt it was time to leave I went up to M (while xH was still standing with him) and asked smilingly if he was ready to go. I then 'made the rounds' to say goodbye to people. Got hugs and 'bye D25's Mum!' from all her friends. OW was chatting with D25's BF's mum and it would have been rude not to go say goodbye to her (D25's BF's mum that is!). So I marched up and gave her a hug and said goodbye. Then turned vaguely to OW and said 'Bye, nice to meet you' with a smile and walked back to M and xH. He gave me a hug goodbye and off we toddled. We laughed all the way home at how easy it was and how weird because it wasn't really that weird.
One final thing that made me happy was that after those first initial moments, it was clear that my girls were feeling relieved. So that's a good thing. They can stop bugging me now, haha. SIL2 called me just as M and I got home. I'd told her I was going to the party yesterday when she called me to invite us to her and BIL's Xmas BBQ next weekend (xH and OW will also be there). Yesterday I told her I'd wait to see how I felt after the party last night. Last night when she called I said we'd definitely come and that I would be completely fine. Ironically I predict that xH and OW will, at some stage, start wishing they hadn't wanted me to be OK with attending the same functions as them.