When do older people become mean and not just rude?

My daughters have both made it to visit their Grandmother who just turned 88. Shes their Dads mother and really the only Grandmother they have ever known. For someone who has been loving and sweet to them all of their life she has suddenly turned problematic on one of them. Its as if one of the girls (and her kids) simply were not welcome there after they bought the plane tickets to visit. On one level I realize that there is something going on her with Grandmothers thinking process but on the other hand its a hurt that I cant seem to resolve for my beloved daughter. I just wish that her memories of her Grandmother werent going to be bruised and tarnished by this summer but I am very afraid they will be.