Aging Parents

Aging parents are a concern for many people, now including me (and my brothers). Since my dad died almost one year ago (Aug. 2nd) my mother's emotional state has really deteriorated. She's afraid to go out for the most part, can't seem to get the hang of starting her car very well, and seems bitter about everything and everyone. What's interesting is she is physically well for 82, and mows her lawn, cooks, cleans, walks a mile a day, etc. However, it's difficult to even talk with her 10 minutes long distance without her going off on something and she gets very angry and hangs up.
My younger brother and his family live in the same town, so he unfortunately bears the brunt of her nasty behavior. This is not the mother I grew up with. She's now in her early 80s, and we know that there are memory issues, and there's an increase in the hostility level when talking with her about anything. She seems paranoid, and is derogatory to those trying to help her. The trick is getting her to admit it - which she doesn't - or trying to get her help. We're exploring options and it's going to be a hard road, whichever one we choose. I just miss the mother I grew up with.