What is the ruling regarding a wife who left her husband because he married another, and went to her family's house, and he has not asked about her or their children for 3 years, knowing that the husband is from her family?

It is not permissible for a woman to abandon her home and desert her husband merely because he married another. This is an act of disobedience, and she must repent and return to her home and children, for Allah has permitted polygyny. Separating from the husband without a valid reason is a grave sin, as the Prophet, peace and blessings be upon him, said: "Any woman who asks her husband for divorce without a cause, the scent of Paradise will be forbidden for her." A wife leaving without her husband's permission makes her defiant (nashiz), and her rights are forfeited. As for the children, they are entitled to maintenance, clothing, and housing, and the husband must provide their past maintenance. The husband should address the issue with forgiveness to preserve the marriage.