Report the sex abuse that is were you are the victim. Depending on the age difference the police and prosecutor may not be too concerned with the stat rape.
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The age difference would be an important factor if the police would do anything anyway. For example if your cousin was 16 or 17, I'm not convinced the police would even do anything. Currently your cousin is 19. That is not much older than 16 or 17.
Under that scenario I'm not certain the "unlawful sexual intercourse with a minor" is your business. In addition you are not a direct witness to the events. Lastly, it doesn't sound like you are a mandated reported anyhow.
Whether you report it is up to you. Be prepared to have to report it and re-report it when you are interviewed again by the police. Be prepared to go to court. Be prepared to testify if needed.
This is less legal help than practical opinion.
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