I'm sorry that you are going through this. The same thing happened to me last year during the planning of our wedding. My in-laws were being jerks about it all and my parents ended up having to pay for everything. They were on a limited budget and they gave us a number of people we could invite from each side. Hubby and I were fine with that. His parents on the other hand kept adding people to the list. We made them pay for any extras and they had to pay for the stamps even to mail the extra invitations off. It wasn't fair to the people we wanted to invite. We wanted our wedding to be very small and personal and they wanted to invite people we had never even heard of. Just showing off.
I had to stand up to my in-laws and I'm glad I did. They didn't make the planning process easy but hubby and I got the wedding we wanted, not what they wanted.