If you take advice from Emily Yoffe, who writes the "Dear Prudie" advice column for Slate, that's what you'll hear. Seriously. (It's the second letter)
If your mother has filled out a ballot for people who are too mentally incompetent to make their own decision, or is she has filled out a ballot that is contradictory to the wishes of one or both and coerced them into signing it—or signed for them—then she is committing a crime. Since the integrity of the voting process is fundamental to democracy, and if your report of your mother’s actions is accurate, then her behavior is morally and legally indefensible and I join you in condemning her. At this point, assuming the ballots have been mailed, the only way to intervene would be to report this to the local election board. They should flag the ballots and at the least see if the signatures on the envelopes match those of the voter registration applications. Maybe the officials will find the ballots suspicious and investigate further. Let's say they decide to make an example of your mother and prosecute her for voter fraud.
Yes, justice may well be done, but at a very high cost to your family. It will be obvious that someone with a close but not affectionate relationship ratted her out. If your mother ends up busy with her legal defense, then someone else is going to have to pick up the slack of caring for your grandparents. Not every wrong can be righted, and I think you’re better off concluding this is one of those cases. But if on Tuesday it turns out the presidential election is decided by a two-vote margin in Ohio, then you’re going to have a story the whole country will want to hear.