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
Barclays has earmarked an additional £700 million to meet payment protection insurance (PPI) compensation claims, it has been announced.
The new communications director attacked other members of Trump's senior team, and threatened to fire his whole department over 'leaks'.
Friday will see sunny spells across Northern Ireland and Scotland, but also some scattered showers turning heavier.