Is a young woman considered sinful for continuing a relationship with a young man who proposed to her, but whose parents refused him due to a difference in nationality, despite her performing Istikhara and feeling at ease, and their agreement to be patient and supplicate, with each of them desiring to marry the other and her lacking family support?

There is no objection to waiting for the parents' approval of the marriage, but the relationship with the man must be completely severed to avoid temptation, for the Prophet, peace and blessings be upon him, said: "I have not left behind me any temptation more harmful to men than women." If the parents agree, then marriage is your appointment, otherwise accept their decision, and Allah will compensate you with something better.