OK so here is the shortest version I can givw
I was adopted at birth, raised by my parents with a large and loving extended family (aunts and cousins etc). Upon reaching adulthood I did seek out bio-parents, largely for the purpose of obtaining medical histories. I have maintained contact with some biological family over the years, I have a half borther on each side. My borthers are invited to the wedding, very few other biological relatives are for both obvious reasons ( i don't have a realtionship with them) and practical (money does become an object at a point).
TODAY my older brother emails me all surprised and huffy that I didn't invite "our" aunt and two cousins (none of whom i have met and at best have exchanged a couple of emails with over the years)....who live 1500 miles away....Is there seriously some expectations that because we have a few chromosomes in common they should have been invited? And even if my borther feels they should have, is he trying to invtie them after the fact? who wants an invitation under those circumstances...I don't get it (the weddings in less than 40 days btw)

Re: Seriously?
My sister was in my BP, and all 3 half-siblings were invited. 2 of them came. I also invited my bio mom, and at the last minute she asked me if she could bring her mother (my bio grandmother) instead of her boyfriend. I didn't want to cause waves, so I said that's fine.
If I were you, I'd go with what PP's said. Just explain it to him the way you would explain to ANYone - space, budget, whatever - that you're very sorry you weren't able to invite everyone you would have wanted. While this is probably not the right time to start setting boundaries with them - if you haven't already, I would recommend you start doing that.
Keep in mind that these people see you as their "long lost" relative. But what they don't understand is that, to you - they're probably just random people that you met recently. You don't have the same emotional connection that they do. They've been waiting 20+ years, wondering where you are and how you're doing. You've been going about your life without much of a thought of them, until now.
It took me several years to find a comfortable place with my relationship with my bio mom (including calling her by her first name, and NOT "mom" like she has asked me to do). But you can do it!
Married!