Connie,
Try to speak to whichever parent of the grand-daughters is your child in private and at a time when emotions are not running high. Express your concern for the older child in a non-accusatory manner. Acknowledge that it is difficult to be a parent to teenage girls and that you know they are doing their best but that you wanted to make sure they realized what it looks like to an outsider.
Maybe they don't realize the dynamic that has take hold.
Do not say anything in front of your grand-daughters. That will cause far more problems.
And I agree with the poster that said you should offer a loving place for the older child to come to. She needs it.
My mother did the same with one of my brothers - always taking his side. That brother still doesn't seem to fit in with the rest of us as adults. She didn't do him any favors, but I'm sure she thought it was the right way to handle it at the time for some reason.