Is a woman's customary marriage to a man who is legitimately married to two women permissible, especially with witnesses and a lawyer confirming its legality? And is it permissible to conceal this marriage from his family and her family in order to preserve their households?

An 'urfi marriage (customary marriage) is valid if it meets the Islamic legal requirements, and notarization in courts is not one of its conditions. However, the presence of a guardian (wali) is a condition. The majority of scholars believe that a marriage without a guardian is invalid, contrary to the opinion of Imam Abu Hanifa. Their evidence is the hadith (definition) narrated by Aisha from the Prophet, peace and blessings be upon him: "Any woman who marries without the permission of her guardians, her marriage is invalid, invalid, invalid." Therefore, what you did by marrying without a guardian is invalid, and the contract must be renewed through your guardian (your brother, then your uncle). If your relatives refuse, you can take the matter to a judge to marry you, based on the Prophet's saying, peace and blessings be upon him: "The ruler is the guardian of whoever has no guardian." And placing the fiancée's hand in the suitor's hand is not permissible before the completion of the marriage contract.