She may be angry because shes lonely.lonely people arent about to tell you that they are lonely because its not accepted in our country.Loneliness is like a weakness to some and it comes out as anger sometimes.Its a shame I know but it is what it is.There are senior centers in most cities.She probably needs a break just like you do.Look into the senior center in your area ,theres others her age that she can relate too,good food for lunch so that she doesnt have to eat alone and company to mingle with.We all need people around who we can talk to even if we deny we need anyone. If she doesnt want to go,stand up and demand she try it and tell her to stop the stuborness