Rsvp's - how to handle too many additional guests

This is very presumptuous! Especially when money and space limitations are involved, they are putting you in a very difficult position. I would have your fiance just be very open when he talks to them, and explain why the limitations on guest numbers exist, and the reason you were only able to reserve 4 seats in their name. Saying something like, "we would love to be able to have all of you there, but unfortunately our budget and venue can't accomodate more than X people" takes the onus off you.

Also, worth mentioning that family members are often not aware of the realities of wedding costs. If they are insistent, you could propose the option of his aunt paying for the cost of her additional guests and let her know exactly what that number is - food, extra tables/chairs, table settings, etc. This will likely wake her up to exactly what she is asking of you by bringing these extra people. At the end of the day, no one can force you to include people that are not the most important to you!

Thank you, I am going to have him explain that to her. She could have at least called before sending the RSVP back and asked us if that was ok or if it would even be possible

Your fiancé needs to call his aunt and explain that you can only accommodate whoever was listed on the envelope. And say it just like that. Don’t say “due budget” or they will try to “fix” that for you. Explain to your fiancé that this would not be “uninviting them” because the 17 extra were never invited to begin with LOL.

17 people?! That's crazy. As PP says you can't "uninvite" people who were never invited in the first place. The way to handle this is to call her and apologize if there was some kind of misunderstanding but that you have her reply card for 17 extra people and wanted to know if that was a mistake. If she says it wasn't or if the reply card was meant to make some kind of point, then let her know that the invitation was meant only for her, his cousin, and two guests. While you don't need to make any excuses, I would probably just say that it's a small wedding.

Don't justify anything if she argues or tries to bargain. If she says it means she won't attend, tell her you are sorry to hear that and will miss them but will have to get together with them and the other 17 ( if they are also close family), on another occasion. Out of curiosity who are these people?

Oh goodness she rsvp'd for 17 people, who are these extra people?! I d def have your fiance or one of his parents call her (whoever's sibling it is) and say you are excited to see her at the wedding but are confused because the rsvp card said 17 people. Clearly state you can only accommodate those mentioned by name on the envelope. Give her a date to please let you know by if she will still be attending and how many of the 4 seats you have set aside for her family you should reserve. Good luck!