Father preventing his son from marrying because he needs his help to take care of siblings

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).”
[al-Noor 24:32]

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.