Is a woman sinful for marrying after her divorce from a husband who does not pray and drinks alcohol, and without the knowledge of her ex-husband and her children?

If a man divorces his wife with three pronouncements, she becomes irrevocably divorced from him (baynunah kubra). After her waiting period ('iddah) ends, she is permitted to marry whomever she wishes. For the new marriage to be valid, the presence of a guardian (wali) is a condition. If there is no guardian, or if he is preventing her marriage ('adil), or is not legally competent, then the Sultan (the judge) is her guardian, according to the hadith: "Indeed, the Sultan is the guardian of whoever has no guardian." Since the marriage took place with the Sharia judge, it is valid.