My MIL would have been PISSED if a family portrait was taken and she was not in it! PISSED! My MIL would rather have a horrible photo of the entire family versus really nice individual family photos. (Unless of course the photo was bad of her, and then out it went.)
My grandmother said, about a million times, I would love a photo of ALL my grandchildren, all together. What she really meant was I want all of my grandchildren, at my house, all at the same time. If she found out this happened, and she wasn't there she too would have been VERY angry.
People love the concept of everyone together for one photo. The photo can turn out awful and I can promise you that the majority of grandparents will think the photo is gorgeous.