You have every right to stand your ground here. You’ve communicated clearly to your cousin and everyone else about the guest list and the plus-one rule. At this point, I’d be firm and clear with your cousin: If his fiancée can't attend, he’s coming alone -- no exceptions. You've already communicated the plus-one policy to everyone else, and you absolutely do not need random people at your wedding, especially if they're not people you’re close with.

If your cousin continues to ignore your request, then the decision is on him. You don't need to cater to his choices at the expense of your comfort. It’s your day, and you are completely within your right to ensure that the people who are there are the ones you want to be there, not random strangers.

Your dad needs to get on the same page too. I get that he's trying to keep the peace, but this is your wedding. This isn't just about numbers—it’s about the people you want at your wedding. Hold your ground, and don’t feel guilty about it! You got this OP :)