All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Whoever vowed to do an act of obedience should fulfill it if he is able to, based on the Hadeeth that reads: “If one vows to obey Allaah Almighty, he should obey Him.” [Al-Bukhari]
If you have not specified a specific beneficiary to receive the vowed money, then there is nothing wrong in paying the debt of your father from it if he cannot pay it nor in offering financial assistance to your brothers and sisters so as to accomplish their marriage if they are poor and needy. If repaying the debt of your father and the money that you will offer to your siblings to help them get married reaches the amount that is due upon you, then this will be alright. If you are unable to do so, then you should offer an expiation like that of breaking an oath for what you failed to do pay in the past. You have to be keen on fulfilling your vow in the future. You should also know that spending in good matters increases the money of the spender, develops it, and does not decrease it, as the Prophet, sallallaahu ‘alayhi wa sallam, said: “Charity never decreases one’s wealth,” “Allaah Almighty says: O son of Adam, spend and I will spend on you,” and “There is no day that comes to the slaves of Allaah without two angels descending and one of them saying: 'O Allaah, refund those who give money,' and the other saying: 'O Allaah, ruin the property of those who withhold it.'” [Al-Bukhari and Muslim]
Allaah Knows best.