Anna Kepner's Brother Allegedly Heard Screaming Coming from Cabin Before Teen's Death

Anna Kepner's ex has come forward with harrowing new details about the final moments of her life ... alleging she was involved in a heated confrontation with her 16-year-old stepbrother before her death.

Joshua Tew -- who described Anna as his first love during a new interview with Inside Edition -- said her 14-year-old brother Connor told him about a nasty fight she was allegedly in with her stepsibling in the cabin they all were sharing aboard Carnival's Horizon ship.

Anna and the unnamed brother were allegedly inside the locked cabin ... and according to Joshua, Connor could hear him "yelling at her in a harmful way." Joshua was told the stepsibling even yelled at her to "shut the hell up." There was also some "banging around" heard in the room ... including what the brother believes were chairs being thrown.

The 18-year-old cheerleader's body was found in the shared room on Nov. 8 -- reportedly wrapped in a blanket and tucked under a bed -- and ABC News reported she died from asphyxiation caused by a "bar hold" after someone’s arm was held across her neck. She is not believed to have been sexually assaulted.

Joshua told Inside Edition Anna was scared of her stepbrother and often slept at friends' homes because "she didn't feel safe around him." He also alleged he was sexually harassing her ... but Anna was too scared to tell anyone.

It's been reported the stepsibling had a strange infatuation with Anna. Joshua's father, Steven Westin, even alleged he once tried climbing on top of her in bed.

The FBI is currently looking into a stepbrother of Anna's in connection to her death, which was revealed by her stepmom, Shauntel, in a recent court filing. Anna's stepmom and father, Christopher Kepner, were on the family cruise when she died.