Is it obligatory for parents to financially support their unmarried working daughter (27 years old) with her clothing and education expenses, knowing that they are able to do so, while continuing to financially support her married brother (31 years old) and her student sister, and justifying their refusal to support her on the grounds that she receives a salary?

If the children have a salary or income that makes them self-sufficient, then their maintenance is not obligatory upon the father. As for the sister who is studying and has no money, her maintenance is obligatory upon her father. Some jurists have stipulated that educational expenses are included in the obligatory maintenance.

If the giving is in the form of a gift, then the father must be just between his children, unless there is a justification for preference, such as the son having many dependents, or the daughter needing to pursue knowledge.