What is the ruling regarding a girl who loves a committed young man and prays to God for him to be her destiny, while he is unaware of her love and tells her that she is like his sister to avoid temptation and because he has not yet prepared himself?

It is not permissible for a man and a woman who are not mahram to communicate except out of necessity and to the extent required, while adhering to Islamic guidelines and avoiding fitna (temptation). One must observe the Sharia limits. A young man's statement that he views a girl as his sister is not a justification for continuing communication, for Satan entices a person to disobedience, as the Holy Quran warned, and the Prophet, peace and blessings be upon him, clarified that the eye, ear, tongue, hand, and foot all have a share in zina (fornication/adultery). As for supplicating for marriage to him, there is no objection to it, and Islamic law has encouraged the marriage of those who love each other. However, it is preferable not to specify a person in the supplication. As for your love for him, there is no sin in it, as long as it was not caused by leniency in interaction or leads to that which displeases Allah.