What is the ruling on marrying a young man after the approval of both families, while there is an older unmarried sister, in a society whose customs require the marriage of the eldest sister first, and what is the ruling on speaking with this young man during this period?

Practices contrary to Islamic law are not permissible. This includes preventing the marriage of the younger daughter before the older, due to the injustice it entails for her and the ensuing discord and corruption. The Prophet, peace and blessings be upon him, said: "If someone whose religion and character you approve comes to you, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." Scholars have issued fatwas (religious edicts) against the permissibility of this prohibition, as it is a custom of the common people that has no basis in Islamic law and causes harm to the younger daughter. Guardians should not allow such customs to be an impediment. Either the lawful marriage contract should be concluded for the suitor with the guardian's consent, or communication between the two parties should be severed, as maintaining such communication is a reprehensible act.