Legal Question in Criminal Law in California

The limitations period restricts only when charges may be filed. It has nothing to do with when a defendant may enter a plea or stand trial. You haven't told us either when the crime allegedly occurred or when charges were filed, so it's not possible to answer your question.

I should add that the limitations period can be extended for a variety of reasons. I believe, for example, that the clock does not begin to run on charges of sexually assaulting a minor until the victim becomes an adult. Your question does not say how old the victim was at the time, which is another reason we can't give you a reliable answer.

The "right to prosecute"? Of course, that is his job. Its a little late to worry about whether there were legal defenses, like Statute of Limitations, that could/should have been raised at the time. Not raised, they are waived, as are untimely appeals. BTW if the victim was a minor, they have up to 8 years after turning 18 to bring a civil lawsuit. The conviction could be used as the 'proof' to win the civil case.

I have to disagree with part of Mr. Nelson's answer. Most defenses can be waived if they are not asserted in a timely manner, but that is not true of the statute of limitations. If the limitations period expired before the case was filed then the trial court lacked jurisdiction to hear the case. Lack of jurisdiction is a defense that generally cannot be waived. If this is indeed what happened, then your relative should still be able to challenge his conviction.