I cannot count how many family members she gets mad at and it has gone on. My mother is 87 and we believe she has alzheimers. Her mother and sister both had it and died with complications. My mother seems to get mad all the time. If anybody leaves, she says she doesn't want to go and she is mad when we get back. She got mad because she thinks my niece got mad at her but that didn't happen. It seems like she is mad about everything.
Your mom has seen her mother and her sister go through terrible things with dementia, and now, on some level, she suspects that is in store for her, too. Wouldn't you be mad? And frightened? Mad might be easier for her to handle.
I hope you can provide comfort and reassurance. "Niece loves you very much. You know she hasn't always been real good in expressing her feelings. I'm sorry she came across to you as mad, but I don't think she really was. She asked about you on the phone this morning. She cares about you." I'm not sure you can talk her out of being mad, but let her know you accept her and love her as she is, and you intend to be around and help her.
As 97yroldmom points out, there may be some medical help for her.