A Caribbean cruise promising sunshine and smiles has ended in devastation for the Kepner family, after their bright and ambitious 18-year-old, Anna Kepner, was found lifeless aboard the Carnival Horizon earlier this month. The heartbreak only swelled as the family learned who investigators have named as a suspect: Anna’s own stepbrother.

Reeling from the tragedy, Anna’s grandparents, Barbara and Jeffrey Kepner, poured out their anguish in an emotional sit-down with ABC News. “We were all having the time of our lives,” Barbara recounted, her voice trembling. “Never in a million years would I believe anyone could hurt my sweet Anna.”

Anna’s future seemed limitless. The fiercely independent teen was just weeks away from finishing high school, with bold plans to serve her country in the Navy, according to her family. But those dreams vanished in an instant when Anna was discovered dead while on vacation with eight relatives. Authorities have kept their lips sealed on how Anna died, but her family says the FBI indicated she was asphyxiated, possibly in a deadly chokehold.

Jeffrey Kepner admitted he looked forward to little more than enjoying time with all of his grandchildren and kids on what was supposed to become an annual tradition. “The cruise was just an excuse,” he said. “I wanted another week with my family together.”

The multi-generational group took up three cabins on the ship. Anna, her siblings, and stepsiblings—even those with no shared blood—chose to bunk together, according to Barbara. “They were more than step-anythings to us,” she insisted. “We told them, ‘If you ever want space, come crash in our room. There’s always a bed for you.’”

For the Kepners, labels like ‘stepbrother’ and ‘stepsister’ simply didn’t exist. “Our love doesn’t have limits. When our son remarried, we embraced his new children as our own. Everyone called us Memaw and Peepaw, and that made us feel complete,” Barbara said, reflecting sadly on the family’s shattered unity. “We loved them all as if they came from us.”

So when the grim news arrived that the authorities had eyes on Anna’s stepbrother, the family’s heartbreak deepened. As the FBI keeps its investigation tightly under wraps, the Kepner family must now grapple with grief and unanswered questions, their once-happy blended family haunted by tragedy at sea.