Ms Martin was called to the stand by the state during the hearing, which is expected to last until the end of the week as Judge Thokozile Masipa considers what punishment to serve the Paralympic champion after his conviction.
The 27-year-old denied murder by claiming that he mistook his girlfriend Steenkamp for an intruder when he fired four gunshots through a toilet door at his home in the early hours of Valentine's Day last year.
When referring to the moment when Steenkamp's family were informed of the shooting, Ms Martin told the court: "[Her mother June] was hysterical. There were people there, helping her to cope. They had given her medication. She was very medicated. She just walking around, shrugging.
"My uncle was sat in the corner, just crying. Crying, crying, crying. It's ruined our whole family. It's ruined Uncle Barry and Auntie June. Reeva was everything to them. They absolutely adored her - they were so proud of everything she accomplished."
Pistorius's defence have called for the judge to impose a three-year home arrest.