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.
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A 24-year-old man has been arrested on suspicion of murder after the body washed up on the Thames shoreline.
Police were called to an address in Carlisle on Friday night but the eight-month-old died later in hospital.
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