Is it obligatory for children to obey their mother if she orders them to sever ties with their maternal aunt and her children?

Kinship ties are obligatory, and whoever severs them is a sinner. If the mother's رضا (contentment) is only achieved by severing kinship ties, then no attention should be paid to that, because it is an act of disobedience to Allah Almighty, and there is no obedience to a creature in disobedience to the Creator. The children can be gentle with their mother and remind her of the seriousness of severing kinship ties, and strive to reconcile her with her sister.