Is it permissible to give Zakat al-Fitr to a sister or a mother who is a university student and does not have a fixed income?

It is permissible to give zakat to a poor sister who has no one obligated to support her; this is considered both charity and maintaining kinship ties. However, it is not permissible to give zakat to one's mother because her financial support is obligatory upon her son if she is poor.