What is the ruling on my retorting to my grandmother and mother that their prayers against my father have not been answered, and my telling my mother not to listen to my grandmother regarding dividing us? And who is more deserving of filial piety and care from my mother: us, her daughters, or her mother, given the impossibility of combining both?

If the situation is as you described, then there is no blame upon you for admonishing your grandmother and mother regarding their imprecations against your father, and their baseless cursing and reviling of him, while observing proper etiquette and gentleness in your admonition. It is incumbent upon your mother to honor her own mother, treat her kindly, serve her, and care for her if she is in need. However, it is not permissible for her to obey her in severing ties with her daughters or in baselessly imprecating against them and their father.