What is the ruling of Islamic law regarding a fiancée retracting from the engagement based on inaccurate information her sister gathered about her?

Verifying the suitor's situation is permissible, and appointing an agent for this is allowed. What your fiancée's agent did is permissible if it was without spying or lying, and your fiancée is then permitted to break off the engagement. However, if it involved spying or lying, it is a reprehensible act, an injustice, and an aggression, and the perpetrator must repent. One should not act upon the news of a transgressor, as Allah Almighty says: "O you who have believed, if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and then become regretful for what you have done." (Quran 49:6) Breaking off an engagement with a legitimate Shar'i justification is permissible, but without a Shar'i justification, it is disliked and blameworthy.