It's ok to draw boundaries around what you will and won't accept. Nobody here will fault you for that. If you can't tolerate being in SS17's presence (and that's not being petty), and your DH knows that and understands why, then he's an ass for just ASSuming you would ignore that boundary in this case.

I can understand DH wanting to include his son in a family reunion. Are there other options? SS drives himself and stays with DH's sister, while you and DH stay in a hotel? SS goes to the event with someone else? Nothing says you have to talk to SS at the event. If you cross paths, a simple greeting and move on is sufficient.

Do you otherwise like his family and the people who will be at the reunion?  If so, maybe consider going, ignore the little twit and show your DD off to everyone and shine as your DH's wife.  I hated the family events with my SO's family.  I stopped attending them.  He can go without me, no problem!

The last place I'd want to be right now, in the middle of a pandemic, is a packed country house with tons of people I don't know or care about who are coming from God only knows where.

I see DH's side of wanting his son to go. However, I also see your side of not wanting to spend time with someone whose mother is looking for ANY excuse to legally nail you, especially if SS has had any part in it.

I think you need to go back to your DH and lay it out for him:

"No, DH, this isn't my choice. I have not been the one to choose that your XW and son treat me with such disdain that I'm in constant fear of what they're plotting against me. BUT, I DO continue to choose you and our little family with DD because I continue to think that we mean something to you. I understand that you want SS to go, and that this is a tough spot that you're in. However, I won't let you make me feel bad for protecting myself when I haven't done anything wrong. I won't let you put your anger on me. YOU made the choice to procreate with BM. YOU also made the choice to allow SS to behave inappropriately. YOU have allowed them both to be awful to me. I get to be angry, and I get to put the blame where it lies - at your feet. So, any attitude you think you have about this situation needs to be resolved elsewhere because the only folks who get to be upset by this are the folks who have innocently hurt, and that's DD and I. You need to figure out how to protect us from your poor decisions in procreating with BM AND poorly parenting SS."

Stop being his emotional punching bag and put it all back on him. Be sad in private, but be a damned lioness to him. Show your daughter that a strong, independent woman doesn't accept scraps of a relationship.

Of course it hurts to be put second because that is not the natural course for a husband and wife. Your DH knows the chaos BM and SS has caused you. He doesnt seem to care to show off his wife and DD at a family reunion. But rather have SS there if he was to chose. That in itself is shitty for you. Why, because you are not the problem but SS and BM are, but you get punished. Seems backwards to me, but then again in this dysfunction whats down is up and whats up is down.

I would stay home with DD. I would go on strike with DH. If any of his family members ask you directly why you werent there be honest and truthful in your answer. Let them know you have been steam rolled by SS and BM and you cannot be around SS because of this. Your DH chose to take SS and we stayed home.

Your DH is making a dickhead move. I would not reward him for that behavior.

Not only would I not go and instead plan something enjoyable, but I would also insist that your H quarantine before coming back into your home.

The only thing worse than him being a d@ck and going without you DURING A PANDEMIC would be him infecting you and/or your bio with COVID.

I agree with the comment above. It hurts because your spouse is supposed to put you before the children. As a non-step example, DH and I have a newborn. DH admitted that he is feeling a little hurt and jealous that I'm devoting most of my attention to our baby right now, even though he understands it's necessary because she's a newborn. If I were to continue to put her first after she becomes more self-sufficient, our marriage will suffer.

For some reason children from "first families" are expected to come first just because they existed first, but it's the same dynamic. BM and DH went on a trip away without SSs when they were married and everyone applauded. DH and I went on a trip away without SSs and BM and members of his own family told him he was a terrible parent. I think there are many who subconsciously believe that if you get a divorce you forfeit your right to a happy adult relationship (esp if you are the divorced dad).