Take your 70-200, and get either a 16-35 or a 17-40. Cover the 40 to 70 with a few steps either way.
If I were there taking pictures of you and family, I'd have the 17-40 on most of the time. Up close and personal with the people, and the backgrounds are all there and in decent focus. Five years from now you'll look back at the pictures, see yourselves AND get a good look at where you were.
If you want a fast prime, stay home and do the blurred-out portraits there!
Enjoy the time with the family!