Is the aforementioned marriage legitimate, and does it entail the necessity of an Islamic divorce?

The aforementioned marriage is void due to the incompleteness of the required number of witnesses, as the majority of scholars stipulated the testimony of two just men, and a woman's testimony is not valid for marriage. If no one attended it except your sister and her husband, then it is void, and the full dowry is obligatory because of the legitimate seclusion that occurred, and the husband must pay it. The dowry of equivalence (Mahr al-Mithl) is obligatory even if it was not specified in the contract. The divorce is valid and binding, and by it, you become unlawful to him, and the non-documentation of it by the Sharia courts has no bearing.