It will be, I think, until the OSCO gets DNA back. The grandparents, remember, have had to deal with DNA before in finding out their grandchild's father isn't who they thought it was. IMO, regardless of what they spew publically, they will know from the DNA. What they won't accept is if Casey is convicted of killing her.
I also saw a couple posts comparing to Mark Hacking's family and how they stuck by Mark. The difference there, if I recall, was Mark was in a mental facility at the time of his arrest and not a jail. Mark's family did stick by him -- after they convinced him he needed to tell the truth.
And to be blunt, Casey was never confronted that way by her parents that we see: in all the video visits, they don't address the issue head on and Cindy has "leading" questions that make the lie bigger based on Casey's responses. It makes Casey's ability to spin stories to her easier and that's why they're in this tragedy they are today, IMO.