Marian Brown was pregnant when she was shot and fatally wounded while walking back to her sister's house along Roden Street in West Belfast in June 1972.
Her sister, Teresa, told a new inquest into her death that a few days later two bullets in an envelope were posted through her letter box.
She was with Marion when she was shot.
She described hearing a burst of gunfire and man shouting at them: "Hit the dirt!"
When the shooting was over, she went over to her sister who was lying on her back with her boyfriend lying across her.
She grabbed Marian's wrists and she said that she knew she was dead.
The Brown family believed at the time that Marian had been killed by loyalist gunmen firing down Roden Street. However,  it later emerged that soldiers had opened fire during the incident.
A fresh inquest was ordered after questions were raised over the velocity of the bullet that killed her.
Last week, Marian's older brother, Richard, said he "felt ashamed that he couldn't do anything" to help her and he was angry that whoever fired the shot "took her life and robbed us of a sister, a daughter and a friend".
Mr Brown said the 17-year-old "never got to bloom".
The inquest is being heard in Belfast by Judge David McFarland.
A sister of a 17-year-old girl shot dead in disputed circumstances in 1972 has been describing the horror of the night she was killed.