What is the ruling on lying to a younger brother to prevent him from an illicit relationship, by telling him true information about another girl's fiancée, but falsely attributing the source of this information to the girl herself instead of her friend?

Lying is to inform contrary to reality, and it is forbidden, based on the Prophet's (peace and blessings be upon him) saying: "Beware of lying, for lying leads to immorality, and immorality leads to the Hellfire." However, it is permissible for a valid purpose that cannot be achieved except through it, provided that it does not lead to something forbidden, such as backbiting, slandering, or any other harm. Euphemisms (Ma'areedh) provide a way out of falling into lying.