Must I confess what I did to my cousin to absolve myself of the injustice?

The duty is to repent to Allah by ceasing the sin of harassing the maternal uncle's daughters, regretting the act, and resolving never to return to it. It is not obligatory to seek absolution from the wronged party in the manner mentioned. Rather, it is obligatory to conceal one's sin and not inform anyone about it, and to strive in supplication and seeking forgiveness; for that is sufficient. Exposing the sin generates discord and anger. Instead, one should turn to Allah to appease the wronged party.