If I reveal this secret to a therapist will I put my Uncle at risk from prison?

Consequences for your uncle are much less important than getting yourself into a better place mentally. He was an adult, and you could not legally consent, based on the fact that mental abilities are not complete until the early 20s. You must be honest with your psychological counselor or it will serve no purpose. In the US, ual crimes against minors can be prosecuted up to about 12 years after the fact, depending on the state law.