After an argument, my cousin showed some private text messages about a 3rd person's physical appearance to our common acquaintances.

You are in a difficult position if in fact you made the statements about the third-party that were revealed to the third party. Truth is an absolute defense to libel and slander. For example, if you said the person does not pay her income tax. And your cousin told the person that you said that she does not pay income tax. your cousin has not liable and slander and this person if the person does not pay income tax. What did you say to the cousin about the person? Is your statement correct?