Is it permissible for parents to prevent their son from marrying on the pretext of waiting for his elder sister to marry?

Islam has encouraged and urged marriage, for it is among the traditions of the Prophets and an act of worship through which one safeguards themselves. The Prophet, peace be upon him, encouraged youth to marry. One should try to convince the parents to agree, but if they are not convinced, you may marry even if they do not approve. This is further confirmed if you fear falling into what is forbidden (haram), and this would not be considered disobedience (uquq).