this reminds me of my inlaws 60th. BIL and SIL, who lived closer, did all the planning and told us what our share would be - $200. We paid it and the topper was, MIL did not believe we had contributed anything.
Families - ya gotta shut up and love em sometimes.
PS - at your inlaws age (70th anniv. - wow!!) don't make it TOO much of a surprise!!
Be sure to find out their secrets for such a long marriage.