What grounds would passport control in the UK have for preventing these girls from boarding their flight to Turkey? Clearly, their names were not on a watch list and they appeared to have had valid passports and tickets. (The one who is said to have used her slightly older sister's passport may have had a close family resemblance but, if not, we all know how sub-human we look in passport photographs.) Security at an airport – where there are thousands of passengers daily – is important but it cannot override the law. As there were no grounds for suspicion, even under the wide-ranging Prevention of Terrorism Act, stopping them from travelling to a popular holiday destination would have been illegal.
The girls would have to be very stupid if they completed an immigration form in Turkey, or anywhere else, giving "jihad" as a reason for travelling. Holiday would be the most likely and acceptable reason. You never used to have to give an address where you would be staying, especially as many tourists travel around, but things may have changed. If they have, no doubt their contacts would have supplied a suitable address.