I haven't been able to find any serious legal analysis of the situation. Many articles just repeat the "50 year" possibility without explanation. Since the nephew apparently was 21 at the time, the entire weight of a charge would rest on the incest allegation, and what Puerto Rico law says about it when it occurs between adults.
I've checked about a dozen articles and not one identifies the relevant statute, and none quotes it either. What to speak of recent history of how the law has been applied. "Law 54" is frequently mentioned, but that governs not identifying domestic abuse victims, not incest.