Police officers and ambulance crews were called to a house at Mornington Lane in Lisburn shortly after 6 am yesterday.
A woman died, despite efforts by paramedics, while another woman and a man were taken to hospital with what are believed to be stab wounds.
The Belfast Health and Social Care Trust said the injured man has since been discharged.
A 19-year-old man was arrested on suspicion of murder.
He remains in the PACE custody suite at Musgrave PSNI station in Belfast.
Local politician said he “can’t begin to imagine the pain” that the victim’s family would be feeling.
“Christmas Day should be a day families spend together, happy and grateful to have each other,” said the SDLP MLA.
“Instead a family woke up on Christmas morning to news of the death of a loved one.
“My thoughts and prayers are with the woman’s family and friends at this extremely difficult time.”
He added: “I would urge anyone with information, however small, about what happened here to bring it forward as soon as possible.”