<human>: I have posted the same thing on /r/wedding but I am still in need of advice, so here goes.

My weddings is less than two months away and the invite/guest list has been a huge source of conflict between my family and my fiance's family. My FMIL and her sister (Aunt) were going out of the country for a month so my family asked them for a complete list of family and friends they wanted to invite. We were under the impression that this guest was finalized and using those numbers, we gave an estimate to the venue/caterers.
FMIL and Aunt return from their trip right before invites are supposed to be sent out and they mention a lot more people who need to be added to the guest list and by the time they are done, the guest list has increased by about 40%. We have finally convinced FMIL and Aunt to stop adding people now that all the invites have been sent but we are now running into another problem. When a couple of RSVPs for his family came back, they had responded for more people than the names written on the invite, which increases our numbers even more. Apparently, this is common practice in my fiance's culture (Vietnamese) that if one person's name is on the invite, their entire immediate family is invited as well. I don't mind people bringing their spouses and grown kids, but this should have been addressed when his family gave us the guest list and the number of people on it. I am worried that our numbers are getting out of control if more people from his family end up doing this, as our estimates for the venue/caterers are way off and we might have enough space for all of these people.
I tried expressing my concerns to my FMIL but she basically told me that theres nothing we can do about guests RSVPing for more people than the ones invited and if we tell people no, then that will cause drama. Needless to say, that doesn't make me feel any better. The guest list can basically grow and grow and I have no control over any of it unless I want to alienate the groom's family. Any advice on this situation?

Condense the text into a few key points
<bot>: Fiance's family is RSVPing for than just the people on the invite and I don't know how to resolve this madness without creating more drama BS.  Help!