Is it necessary to tell one's niece the details of her mother's marriage and divorce from her father, or is that considered revealing what God has concealed, and it might do more harm than good?

If the urfi marriage was valid or if the man believed it to be valid, then the daughter is attributed to her father. If he did not believe it to be valid, then she is not attributed to him according to the majority of jurists, unless he acknowledges her as his child. His renunciation of her custody indicates his belief in the validity of the marriage or his acknowledgment of her as his child, thus making her his legitimate daughter. The girl must be informed of her father and his whereabouts, if known, for the rulings of paternity to apply. As for her asking about the reason for the divorce, she should be told that it is a normal matter. These questions may stem from her need for paternal and maternal affection, and the parents should be reminded to fear Allah regarding their daughter by maintaining ties and providing care.