OP here to answer/address a few comments I've seen.
In the right circumstances, I have no problem with adopting the mindset of the more the merrier and have done so in the past. I used to host a big annual party at my house and encouraged everyone on the guest list to bring as many friends and coworkers as they wanted, just please give me a headcount in advance so I could account for food and beverages. But, to me, there's a big difference between a house party with people coming and going all night vs. a quiet family dinner where you will be seated and eating off of China. I'm much more comfortable meeting someone for the (more or less) first time in the first scenario than the latter.
The issue isn't that we're talking about a teen romantic interest. If my SIL wanted to bring a coworker or my BIL one of his drinking buddies, it would still feel intrusive to me.
We love our baby to bits, of course, but my DH and I are not the type of parents to think the rest of the world cares one iota about our kid. Or so we thought, but my DH's family has surprised me by seeming genuinely enthusiastic about wanting to celebrate this baby's existence and they all, nieces included, frequently reach out to us to arrange a time to get together so they can see him. (Which I don't understand, because I've never cared one fig about being involved with anyone else's kid but I guess some people actually do enjoy them. Who knew?
) Anyway, my DH and I would've been been fine not having a party at all, cutting a piece of cake for the baby after dinner, and calling it a day. But we started thinking the grandparents might be disappointed not to be invited to celebrate their grandson's first birthday, then we didn't feel right inviting my DH's parents but not his siblings. We had already skipped doing a baby shower, "meh-ed" my MIL's idea of doing a meet-the-baby party for the extended family, and weren't able to make it to the big family Christmas this year. So yeah, we kind of felt like
should have a party for his family, but we agreed to keep it to the immediate family only because we don't have a lot of room and we wanted to keep it simple. I wanted to feel like I was having a relaxed evening with family, not that I had to be "on" and entertaining. For me, less people = less stress.
Last night I was trying to think about it from the other perspectives involved. When I was my niece's age I was also in a year long relationship with my high school sweetheart and it never even crossed my mind to ask if he could come to family events/holidays, so this idea of teens bringing their boyfriends along to every family gathering is a new concept for me. I could see inviting the boyfriend if the gathering is taking place in your own home, but to drag him along to Aunt Tipsy's baby's birthday party? Why on earth would he even want to suffer through that? Then I put myself in my SIL's shoes and nope, no way would I ever think it was appropriate to ask to invite someone the hosting couple doesn't know into their home. So that exercise wasn't very helpful.
What
was helpful was realizing I'm now in an uncomfortable position regardless, by either saying he can come or saying no, and the only thing that can be avoided now is the possibility of making my niece and/or SIL feel uncomfortable by saying no. Which means we'll end up saying yes even though my DH and I are still not thrilled about it. (But, of course we'll be welcoming and polite to all involved, no worries there!).
I just spoke to my DH about it again and he was getting ready to text his sister and say it's okay. He'd held off on replying to her last night because he still wasn't fully on board with the idea even though "Everyone on the internet says we're wrong, honey!"
Today I'll wash another place setting and try to dig up another chair.
Thanks for all the comments!