The stepbrother of Anna Kepner -- who was found asphyxiated during a family cruise vacation -- was arrested this week and appeared in a Florida courthouse Friday to face a judge, TMZ has confirmed.

According to a law enforcement source ... the 16-year-old stepbrother was led by U.S. Marshals into a Miami federal courtroom for an appearance ... but since he's a juvenile, the case was sealed. It's unclear if he has been charged in a federal complaint or indictment.

After his court appearance, the stepbrother was released to the custody of a guardian, according to ABC News.

On November 7, Kepner's body was reportedly discovered inside the cabin she shared with her stepbrother on the Carnival Horizon. She was allegedly asphyxiated after someone put her in a chokehold.

Just after her death, media reports surfaced claiming Anna’s stepbrother had a strange infatuation with her. Steven Westin, the father of Anna’s ex-boyfriend, told one outlet the stepbrother climbed on top of Anna's bed on a single occasion.

We've reached out to the feds for more information ... so far no word back.