How to invite someone after the fact

I think if it's your FMIL and FBIL, that would be fine. I wouldn't mail an invite or anything, just tell them that they are allowed to bring a +1. With it being family, I don't feel like it's a huge deal

I don’t think this is the same kind of b-listing that is typically frowned upon. I think it’s fine as long as you have room for both of them and not just one.

I don't think it's the same as b-listing since they are new significant other's for immediate family members. I'm sure they would just be happy to have them included.

I agree with Caytlyn - this isn't really B-listing in the way that's rude. It's just making room for new relationship and trying to be accommodating - which is always appreciated! I think if/when you get 2 no's back I would immediate reach out to both and let them know they can bring a plus one if they'd like.

I agree with PP’s. It’s a different scenario and it sounds like they wouldn’t be offended.
As far as photos, you don’t have to have your FMIL’s bf in all pics, maybe just a few.

I would definitely invite them. You don’t have to be as formal but you can simply let them know that their SOs are welcomed to attend. It wouldn’t seem rude. Actually, it would seem considerate and that you actually cared enough to include their SO.

I would just let him know his girlfriend is more than welcome at your wedding! I don't think you're being rude at all!

I agree with PPs here, it's not rude to now allow them each a plus one. As for photos, just because FMIL brings her boyfriend, doesn't mean he has to be in the pictures. Maybe in a few, if that's what FMIL and you all want. But just having him as a guest doesn't mean he's committed to being in pictures. This is your wedding, you decide who gets into your photos.

I don’t think that is considered b-listing. For example, we are having a 60 person cap as well. My fiancé is inviting his cousins which are our age, but we are not extending plus ones to them. We plan on letting them know that if we have people who decline (which I’m positive we will have at least two) we will extend a plus one to them. I wouldn’t feel bad about it, if anything, it’s a good thing! As for the pictures, they don’t necessarily have to be in them. The only reason I was in family photos at my FSIL’s wedding back in May is because we are engaged.

I don't think it's rude at all, I would actually say it's more considerate. Letting them know that they originally weren't given a +1 because they were not in a relationship at the time, but now would love to have both of them be a part of your special day.

As for the photos, I am in a similar situation as my FH's dad is currently dating someone. Our photographer let us know she will allow her to be in one picture but other then that it is immediate family and bridal party only.