You are arguing with your cousin and getting frustrated because she does not seem to understand what you are saying. instead of saying, "you are not listening to me", you state, "i don't think i'm being clear". you have used what principle of effectively using language

Somehow but I'm not sure

Courtesy

Explanation:

Effective communication is the ability to convey words in a manner that ensures that the purpose of communication is achieved. Courtesy is one of the 7Cs of effective communication. Courtesy means being polite or respectful towards others. In communication, you are courteous when you use words that seeks to better a tense situation rather than worsen it. In the question above, using "you are not listening to me" would have worsen the situation. A softer or calmer wording of the intended message also served same purpose while calming the situation.