What are the sins committed by the questioner and how can they be expiated, after she conveyed to her uncle كلامًا (words/information) about a girl he wished to propose to, without verifying its authenticity?

If you told your uncle definitively that the woman speaks with men, without having knowledge, then you have erred; because it is not permissible to accuse people based on rumors. However, if you conveyed what people are saying with the intention of advising your uncle, without being definitive, then there is no blame upon you. Rather, it is part of the obligatory advice; because backbiting is permissible in six situations, including seeking advice regarding marriage.