Take it from someone who has interfering inlaws (although I love them dearly)....please have the "guts" to say something for the sake of your own sanity!!! I had the same type of situation and things never changed till I opened my mouth. I was always afraid to make waves, so to speak, so I kept my mouth shut for quite some time. If you keep things bottled up you tend to explode...exactly what I did. I apologized and we're on good terms now, but I wish I had taken the opprtunity to tell them how I felt in a more posative way. As soon as I made my feelings known things changed dramatically. When you talk to them remember this (something I recently read in a parenting magazine)...posative, negative, posative. Say something posative, then make your feelings on the subject known (yep, that would be the negative comment), and end it with another posative. For example: Thats so sweet that you guys would throw a party for my little one, but I'd really like to do it myself. But thank you so much for the offer, its so thoughtful of you!
I know what you're going through and I know its hard, but trust me...nip it in the bud as soon as possible!!! You'll be SO glad you did! :)