OH MY WORD!!!! Someone needs to get some backbone in that family along with a voice and use it!!!
1. Get all the kids together - whether it be on a conference call or skype or SOMETHING and ASK THE PARENTS POINT BLANK WHAT THEY WANT TO DO - then RESPECT THEIR WISHES!!!
2. If your cottage isn't big enough for everyone - then don't offer it.
3. THEY SHOULD NOT HAVE A PAY DIME FOR THIS CELEBRATION!!!! It's their 50th wedding anniversary for Christ's sake!!
If they have a lot of friends - send notes out to all of them asking for pictures and memories - this is what we did for my parents 40th - it was a WONDERFUL surprise for them. We did it alphabetical order and put it in a beautiful scrap book - my parents LOVE IT!!!
We had a party at my sister's church and about 50 people were able to attend (my parents got married in January).
This is NOT about what the kids want - it's what the PARENTS want. Tell your SIL to back off and tell her this is NOT about her - it's about her parents. if she doesn't like it - tell her to pound sand. You will tell her where to show up and how much to contribute. JUST TAKE CONTROL. I hope to God you did not allow her to change your wedding or your bridesmaids dresses!!!
If the parents are refusing to speak up then seriously - have a heart to heart with them - find out what THEY WANT - then do it.