Sister City Transparency Act This bill requires the Government Accountability Office to conduct a study of sister city partnerships between U.S. communities and foreign communities in those countries that score below a certain level on a public sector corruption perception index. Elements of the study include assessing the extent to which such partnerships (1) create vulnerabilities to malign market practices, (2) diminish freedom of expression, and (3) allow foreign communities to realize strategic objectives that hinder U.S. economic and national security interests.