How can one deal with two paternal aunts who are shunning their niece and refusing to communicate with her unless she slanders her mother, her family, her stepmother, and her father? And is this behavior considered a severance of kinship ties on the part of the niece?

Continue as you are, for the severance is from your two aunts, not from you, and the sin is upon them. It is not permissible to please them by disobeying Allah, and obeying your mother and her satisfaction are more worthy and obligatory. Thank Allah for His guidance and enablement, and do not despair of reconnecting with them and trying to please them in a righteous manner.