Hi ladies, I'm new here

I have a question about how I should go about my FMIL's new guest list. My parents are paying for everything. FI's parents have never mentioned contributing. FMIL just sent the "finalized" guest list to my mom and the guest list has doubled...
Now, the venue is about $150 a person so doubling the guest list is a big deal. I don't know how to go about this. I cannot ask my FI to talk to his parents as he is currently in Afghanistan. I do not have the best relationship with her and would prefer to avoid any further confrontation with her (few comments a few weeks ago practically ended any desire I had to "get to know her" even though FI and have have been best friends since we were about 6). Anyways, my mom is NOT happy, and dad will be even more upset once he gets home from work and is caught up. I don't know what to do. I don't want to create any drama between our families, especially with FI gone for now. I suppose the best thing to do is let her know the budget was set with the original guest list, any one additional, she will need to pay for? Does that sounds fair? Any other suggestions?
Thank you in advance
