First Cousin Plus One

The cousin's boyfriend should definitely be included (by name on the invitation). The single cousin who is the same age (or any age) does not need a plus one.

Significant others are not plus ones. They should be invited by name. I can't imagine not inviting a SO who has been with my guest for THREE YEARS. Really you should be inviting anyone in a relationship regardless of if they live together or how long they've been together. It's not your job to judge how serious someone else's relationship is...
As far as the other cousin and anyone else not in a relationship, it is your choice whether you want to extend the plus one. If they dont know a lot of people I would 100% give them a plus one. But if they'll know a lot of people it's a little more of a grey area.

Anyone in a relationship should be invited with their s/o. You don’t get to decide whose relationship is serious and whose isn’t.

If you have met her boyfriend and spent time with him (birthdays, holidays, etc) then I think you should. Other cousins the same age who are not in relationships do not need plus ones.

I would give a plus one to the cousin in a relationship but not the one who is single. That’s our rule, at least.

We're adhering to the "no ring, no bring" rule for our cousins. Even though yes some cousins may be in serious relationships, we've never met them. For example one of my cousins (who is local to me) had been dating a guy for over 5 years and I've never met him, I don't even know the guy's last name!

I think you have to invite spouses and fiances. anything else is optional

If she’s been in a serious relationship for that long I would invite her significant other (with his name on the invitation). I wouldn’t consider that a plus one since they have been together for an extended period of time. Any cousins that are not in a relationship I don’t think require a +1 at all. We invited significant others but did not give any family members a +1 if they weren’t in a relationship.

Yes 3 years is serious he needs a plus one. People choose not to live together for man reasons,

I've been engaged for 2 months and I've only been with my fiance for 2.5 years.
I would absolutely be offended if he or I were left out of a family event because we aren't living together yet.
Put his name on that invite.

Definitely invite him! I was in a smilie situation when I was with my ex. We had been dating about 8 months and were what I considered pretty serious. He had met my entire family and had been a part of holidays, birthday parties etc. We had a family member get married and did not offer me a plus one, but offered one to my first cousin who was not engaged or living with his girlfriend either, they had just been dating a year or two at the time. I didn’t even end up going to the wedding.

Its not up to you to judge the seriousness of their relationship. If all the other cousins get to bring significant others, I would imagine this one would be pretty hurt if she couldn’t bring her boyfriend of three years just because they don’t live together yet. They could end up engaged soon and her boyfriend will be part of your family It’s not worse the tension that could arise honestly.

I'm going to go against the majority here and tell you that no, you don't have to invite their significant others because they can attend the wedding with their families. We are in the same boat as you. Our guest list was maxed out at 220 and both myself and my fiance have large families. I did the math and if we gave every cousin a "plus one" that would be an additional 27 people. We addressed the invitations to our aunts and uncles as "The Jones Family" even for adult cousins. I get that this might be an unpopular opinion but its what we did. If it came down to choosing my 19 year old cousins boyfriend who I've never met or my college roommate, I picked the person I actually knew. We are getting some flack for it, so just be prepared.

Tough choice, so my first cousins, I have 2 I've known my whole life. When we get married we they will be 15 and 20. The 15 year old most definitely will not get any plus or guest and the 20 year old has been with her BF for a very long time and he's invited. Two years ago, we found out my poppop, my dad's father, had another daughter which she has 2 daughters. We've all gotten together recently except my poppop since he passed away 2 years ago. I was thinking of inviting his daughter, her bf and then the daughters 2 daughters with the one's husband and the other daughter who ever she's dating, if dating anybody. Since they really don't know anybody there, that would be great giving them a plus one. I would say just go for inviting their significant other unless they're really young like my cousin haha.

You should definitely invite the boyfriend (by name...he is not a "plus one"). you do not need to give the single cousin a plus one.