What is the ruling regarding an unveiled girl passing in front of non-mahram men, under the pretext that the man covers his face to allow the girl to pass?

These relatives are like any other non-mahram individuals; they must be dealt with according to Islamic guidelines. The Lawgiver has forbidden their entry upon a woman. The Prophet, peace and blessings be upon him, warned against "al-ahma'"—who are the husband's relatives, and some scholars opine that they also include the wife's relatives—because evil is more likely to be expected from them.

There is no harm in a woman passing in front of a man while wearing her hijab, provided the man does not see her. However, the problem lies in being lenient in interacting with them through laughter, joking, and moving around easily, which could lead to forbidden mixing.

One must repent, cease such actions, and observe the Islamic guidelines in dealing with them.