A baby girl is in a "critical but stable" condition after a stabbing attack that killed her brother and sister.Surgeons operated on the six-month-old child on Saturday night.Her five-year-old brother and four-year-old sister died in hospital on Friday after being found injured at their home in Carshalton, southeast London.Police questioned a couple believed to be the parents of all three children.They released the 39-year-old father without charge on Sunday but the mother, 35, remains in custody.A spokesman for the Metropolitan Police said officers were called to the house at 10.30pm on Friday night.He said: "Police and London Ambulance attended and found three children suffering serious injuries, believed to be stab wounds."We believe we know the identity of the deceased. However, we await formal identification and confirmation that all family members have been informed."Post-mortem examinations on the two victims are expected to be carried out early next week.The two older children were taken to hospital but both died within 90 minutes of the police call. The girl died at 11.41pm and the boy 15 minutes later.The couple's £300,000 two-bedroom property in Park Lane was taped off on Sunday as officers carried out an examination of the scene.Neighbours said the couple were of Tamil or Sri Lankan origin and had moved to the suburb in the last year.