Airlines will also face civil penalties, including forfeiting the right to land in Britain, if they fail to provide passenger and crew information.
The London girls, who were friends at Bethnal Green Academy in east London, flew to Turkey last month and are believed to have crossed into Syria and to be living in the Isis stronghold of al Raqqa. There are fears they will become "jihadi brides".
Police gave letters to the three girls, as well as four others, to give to their families about the schoolmate who had already gone. But the girls did not pass them on, raising questions over why police did not send the letters direct to the family homes.
Shamima's sister Sahima Begum, Amira's father Hussen Abase and Kadiza's cousin Fahmida Aziz said they would have done more if the families had received the letters.
Ms Begum said the family was "never given the opportunity" to question Shamima about the other girl who had gone to Syria, finding out about the letter only after the teenager disappeared.
Tasnime Akunjee, the solicitor for the families, said handing out the letter to the girls could have prompted them to travel to Syria. "It is precisely the failure to communicate this key piece of information that disabled the family from intervention in the children's plans. All the police had to do was use an envelope and we might have avoided all of this."