How can I stop my mother-in-law from inviting too many guests to the wedding?

My future in-laws belong to a church full of nosy people. My parents are not Christians and are frankly very wary of church folks. My family has few friends, but all of them are very close to us like family members. All the close family friends from my side probably total less than 10 people. If my mother-in-law tells everyone at her church that they can come to our wedding, my parents will be horrified and very awkward. The thing is, the wedding is AT their church, and you can't really stop her from inviting people since the wedding ceremony itself doesn't cost much-- just the reception. Any ideas on what I should do about this? I want her to be able to invite SOME close friends; just not everyone. I want to make it into a family and almost-family only affair, but he wants our friends to be able to come if they want to. I just think it would be easier if we did it that way.