Praise be to Allah.
It is not permissible for the father to prevent his younger daughter from getting married, if he has received a proposal for her, on the grounds that the older daughter must get married before her. Rather this is one of the customs of the ordinary people for which there is no basis in Islamic teachings, because they think that this is detrimental to the older daughter. But if that were true, it would also be detrimental to the younger daughter. “Harm cannot be warded off by another harm.”
Al-Muntaqa min Fataawa ash-Shaykh Saalih al-Fawzaan (3/152).