The stepbrother charged with raping and murdering 18-year-old Anna Kepner received a glimmer of good news amid all the bad news in his indictment ... prosecutors say there's no chance he'll be put to death, because it's unconstitutional.

Here's how it all breaks down ... the 16-year-old defendant, identified in court docs as "T.H., a minor," is charged with first-degree murder and aggravated sexual abuse, and he faces life in prison if convicted. In the charging docs, prosecutors indicated this is not a death penalty case.

In their new motion seeking to keep T.H. behind bars pending his trial, the feds says the death penalty is off the table under Supreme Court precedent that bars the imposition of capital punishment on juvenile offenders. T.H. was 16 at the time he allegedly killed Kepner ... though he was actually indicted as an adult by a federal grand jury.

According to court docs, Kepner was on a Carnival Cruise vacation with T.H. and other family members in early November 2025.

As the ship was sailing back to Miami from the Caribbean, T.H. allegedly raped and asphyxiated Kepner in the stateroom they shared. Housekeeping staff later found Kepner's body stuffed underneath her bed.