Father preventing his son from marrying because he needs his help to take care of siblings
Praise be to Allaah.
If the son will commit haraam acts if he does not get married, then the father has no right to prevent him from getting married for the reason stated in the question. Allaah is the Provider and those brothers and sisters are part of His creation; He will never forsake them. Allaah has promised to help the one who gets married in order to keep himself chaste. He says (interpretation of the meaning):
"And marry those among you who are single (i.e. a man who has no wife and a woman who has no husband) and (also marry) the saalihoon (pious, fit and capable ones) of your (male) slaves and maid-servants (female slaves). If they be poor, Allaah will enrich them out of His Bounty. And Allaah is All-Sufficient for His creatures’ needs, All-Knowing (about the state of the people)."
The Messenger of Allaah said: "There are three to whom Allaah’s help is due: the mujaahid who goes out to fight for the sake of Allaah; the slaves who makes a contract with his master to pay installments towards his freedom; and the one who gets married because he wants to remain chaste." (al-Tirmidhi, no. 1566. Abu ‘Eesaa said: this is a hasan hadeeth. See also Saheeh al-Jaami’, no. 3050).
But if the son is able to be patient and give himself up to working and helping his father to provide for his brothers and sisters, then he will be rewarded, for Allaah never allows the reward of those who do good to be lost.
And Allaah knows best.