Did the joint will leave the estate to the other grandparent and go on to provide that it would go to your mother on the second death?
What your grandmother has done is change the will after the whole estate passed to her. I have to tell you that she was free to do that. Assuming she was of sound mind the second will would be treated as valid and supersedes the first. That is the typical position based on what you have told me but you should always have the actual paperwork looked at by a solicitor locally to confirm the position. I hope that helps. Please leave a positive rating so that I am credited for my time.