That's awful, Lou. I understand how hurtful it is. Dementia could certainly be part of the problem. In the months before they passed, both my mom and my aunt would make some bizarre assumptions about me and things I was doing (not!). There are a whole lot of reasons why people will twist the truth.
You know the truth, however, and it sounds like your dad does, too. People who saw how lovingly you cared for your uncle will know the truth, regardless of what your grandmother might say.
So sorry for your hurt.