Is it permissible to give a portion of a vow (nadhr) to help sisters prepare for marriage, if the salary is low and there are other commitments and debts?

Whoever makes a vow of obedience must fulfill it to the best of their ability. If no specific recipient for the vow was designated, then there is no harm in using it to pay off your father's debt if he is in financial difficulty, or to help your siblings get married if they are poor. If you are unable to fulfill the vow, then you must offer an expiation for breaking an oath. Spending in good causes increases wealth and does not diminish it.