"She has a right to know why I won’t be back"
Umm no she doesn't, and she will know whether you tell her or not.
She makes no effort to see your family, and even though she has a grandson who is seven and has autistism she makes zero effort to learn anything about autism either in general or how it impacts her grandson.
Your mother puts almost no time or thought into your or your family. Your own time would be better spent doing almost anything else.