A 16-year-old boy has been charged with abusing and murdering his stepsister during a family vacation on a Carnival cruise ship in November, prosecutors announced.

The US Department of Justice alleges the boy "sexually assaulted and intentionally killed" 18-year-old Anna Kepner while the Carnival Horizon was in international waters.

The stepbrother, whom authorities did not identify, was initially charged as a juvenile, but has since been indicted as an adult on first-degree murder and aggravated sexual abuse charges, prosecutors said.

Kepner was a cheerleader who was expected to graduate from secondary school in Florida in May and wanted to join the military, her family said.

Her body was found under a bed, wrapped in a blanket and covered by life vests, ABC News reported.

The Miami-Dade medical examiner's office determined she had died of "mechanical asphyxia".

She and her stepbrother were travelling with multiple members of their family, including her father and stepmother. Kepner's grandparents also were on board.

"We were all having a great time," her grandmother Barbara Kepner told ABC News late last year. "I couldn't fathom why anyone would wanna hurt my baby."

The grandparents had said that the relationship between Anna Kepner and her step-siblings was a happy one, adding there was "no such thing as steps".

"It's all family. It's a blended family, yes, but that's not how our family is," grandfather Jeffrey Kepner said. "Our dynamic is we're all just family."

If convicted, the stepbrother could face life in prison, the justice department said. (BBC)