I don't think for a second that this is intentional. You need to talk to your dad about this, assuming that he is alive. Why wouldn't he be the one to be in regular touch with you anyway? Why wouldn't he be the one to call you and mention that they'll be visiting nearby?
I think that the death of a child, even an adult child, is just about the worst thing that any of us could ever go through. I think that what you are seeing is an unconscious manifestation of her mourning. My best friend died when we graduated college and her mom wasn't her normal self for years. My brother just died and I dread seeing what his passing does to my parents. Grief is a terrible thing that can change us in ways that are hard to recognize.
This doesn't mean that behavior is OK, but try to be as generous and compassionate as possible in how you interpret this. Have a frank, open and caring discussion with your dad and see what his insight is. Once you have his perspective, you can then know how to proceed with gently getting back into her life. And I'm sorry for your loss as well and hope that you and your dad can work together to help heal the family.