What is the Islamic ruling on not adhering to a promise to memorize some chapters (surahs) of the the Qur'an, even if it's only one verse each day?

If the pledge of the questioner and her sister to memorize the Quran did not reach the level of a vow, then there is no sin upon them for not fulfilling it. However, it is preferable to fulfill a promise, especially in virtuous matters such as memorizing the Book of Allah. But if the pledge was a vow, then it must be fulfilled, based on the Prophet's (peace and blessings be upon him) saying: "Whoever vows to obey Allah, let him obey Him."