Is it permissible for a sane virgin woman to marry without a guardian or witnesses if it takes place with an offer and acceptance from both parties, and with the subsequent knowledge of the husband's brothers? And is it permissible for her to marry someone else without a divorce if this marriage was invalid?

This man must be separated from, either by annulment of the marriage (فسخ النكاح) or by divorce (الطلاق), because the marriage that took place without a guardian (wali) or witnesses is invalid, a matter upon which the four madhhabs have reached a consensus. Caution must be exercised to avoid repeating such an action in the future.