First, I recommend no PDAs, of any kind. None.
This is a situation for Miss Manners, regardless of relationship status. This is a family reunion, not your coming out party. In social situations, it is our job to make other people comfortable. If you have news that has the potential to make other people uncomfortable, (and I'd say this could be close to the top of the list of thing that make people who've never heard of it uncomfortable) then that news is best delivered under some other context than the family reunion.
On the other hand, I see your point. Family reunion is for catching up. You want to tell them who you are.
"Whatcha been up to?" "Polyamory. This is my fiance, and this is my lover, who will probably be my first baby-daddy." Doesn't matter how you find to soften the words, that has huge potential for major discomfort.
I understand wanting people to know, I really get it. Both my men were previously my mono-boyfriend, and my parents knew them both. It wasn't a huge leap to say 'I'm with both of them now.'
But I did have similar situation. I changed my legal name (all three, and for the second time in my life) and my parents got very upset about it. They didn't want me to tell my siblings at Thanksgiving. I said I wasn't going to lie, so I'd just be skipping thanksgiving that year. Well, that wasn't good for them. So, I think (*I actually cannot recall now) that we worked out the compromise that I would tell them beforehand, so they would know, but the day would not be about me. Funny, I can remember the hurt about it, but can't remember how we worked it out. The turkey was great!
I understand you want acceptance, but you, sadly, don't get to dictate that. You can do your best to present it as a non-issue, and hope that people take it that way. But you know your family, you have a pretty good idea what people may or may not think about it.
Frankly, it's not their business who your baby-daddy is or isn't; and it's definitely not their business what you do in bed. Do you want to know what Aunt Lucy is doing in bed with Uncle Frank? I bet you'd rather not.
I, myself, wouldn't see it as a 'lie' to say, 'this is Lady, my dear friend' perhaps with emphasis on 'dear.' If you continue bringing both to gatherings, family will figure out you're all a team anyway.
It is a balance, and you have to weigh, how much do you need to show people who you are, versus how much acceptance to stay in your family do you need.
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who lives in the apartment building next door)