Is what the questioner did, by informing her brother that the woman he wants to marry is married " عرفياً" (according to an unregistered customary marriage), considered backbiting (غيبة) or slandering (نميمة), knowing that she is 100% sure of the accuracy of the information?

Backbiting is forbidden by Islamic law and is considered one of the major sins. However, it is permissible for a legitimate Islamic interest in specific situations, such as seeking redress or offering consultation. Therefore, what the sister mentioned to her brother about the woman, with the intention of offering advice, is not considered forbidden backbiting. Rather, it is part of the duty of giving advice. She is obligated to mention the flaws of anyone with whom a meeting is intended for marriage or similar purposes. In giving advice, it is sufficient to mention only what achieves the purpose. If mentioning shortcomings explicitly is necessary and unavoidable, then there is no harm in doing so. As for an Urfi marriage (customary marriage), if it fulfills the conditions of a guardian, witnesses, proposal, and acceptance, then it is valid; otherwise, it is null and void.