Update on this post:

https://www.reddit.com/r/polyamory/comments/117dukr/my_fathers_birthday_party_is_being_planned_and_im/?utm_source=share&utm_medium=android_app&utm_name=androidcss&utm_term=1&utm_content=share_button

Not very dramatic. Boring and uninteresting post incoming.

I talked to my brother, who is always willing to take a fight for better or worse, and he chose to stand behind me this time and support my right to be accepted.

He also talked to my father's wife while I was still trying to decide how to bring it up without offending, in a much more direct and combative way than I would have been able to, and she said that everyone is welcome as long as we're there to celebrate my father, which is a given.

So we all went there as a family and my father greeted my newer partner with "You're the one who lives with Random_silly_name and [older partner]? I've only seen you in pictures. Welcome.". My newer partner confirmed, wished him happy birthday and that was it. They also exchanged a few more words later during the evening but got interrupted by another guest wanting my father's attention. No drama. My partners sat next to eachother and spent most of the evening hanging out together and with a friend who was also there (with assumptions being made, perhaps, but whatever).

So overall, a huge relief after all that worry because of my father actively refusing to accept my choice and acknowledge my newer partner for years. If he can accept "someone who lives with us" as an important person in my life, that is perfectly enough for me. I had to put some thought into avoiding PDA because I didn't want to provoke but that's a small thing.