I feel a breakdown coming on...

FH and I have known each other for 15 years and have gone out for 6. I was never really close with any of his family (he's not either) so we never really dealt with them much. When it came time for the guest list I asked her a few times for a list of the full names and addresses of the people she would like to have invited. I have never really been a big fan of her so FH will go over to see her by himself most of the time. He would come home with a little piece of paper that she ripped off something else with a name - no address. Next time it would be an address - no name. Then FH and I go over there one Sunday afternoon to get all the details and she tells me that she is sick of talking about this. That she will give me what I am asking for, but she doesn't want to talk about it again. Now, you are probably asking yourself - why was I pushing so hard??? I just felt so bad for FH that it would end up being all my family (I have a huge family) and our friends and no represenatation from his side. I don't even want to talk to her, but now I am having major issues with the response cards from her side of the family and her friends. The cutoff was Sept 9 and we are still waiting to hear from 24 people. His sister stopped by our apartment last Tuesday (because she needed something, of course) and says to me 'Oh, I think so and so are coming to the wedding'. I told her that her mother was supposed to get back to me with that person's address and never did and I wasn't going to bother wasting my time so we didn't send them an invitation. So his sister tells me that her mother must have invited them over the phone. What is this a freakin backyard BBQ?!?!?!
So I calmly gave his sister a list of who was invited that hadn't sent back their card and asked her to give it to her mother and have her call them to find out if they are coming. Well, that was last Tuesday - it's Sunday!!!! She retired!!! What is she doing all day!!!! I made FH call and all 3 times no one answered the phone. She is outta control and making me lose my cool! We are already over our limit so I wish I could just consider the non responders as 'no's, but who knows who else she invited OVER THE PHONE!!!
OK - I'm having trouble breathing so I need to stop telling this story. I have to figure out what to do because the next step will be to go over there and smack her upside the head. I need advice.
HELP! HELP! HELP! HELP!