The 17-year-old girl and the 28-year-old woman stabbed in a house in south London are sisters, police said.
The elder sister died as a result of her injuries. Speaking at Brixton Station this afternoon, Detective Inspector Will Reynolds said:
"The lady who was fatally stabbed is the sister of the 17-year-old girl who tried to protect her."
The teenager suffered stab wounds to her head and hands.
More top news
The child died on her injuries and another 14-year-old girl was injured after the buggy they were travelling in was involved in a car crash.
Cloud thickening from the west throughout the day. Rain in northwest later.
The Songs of Praise presenter has voluntarily agreed not to appear on BBC whilst an investigation takes place.