A boy of 14 was stabbed to death in what might have been revenge for a youth who died falling from flats as he was chased by a gang.
Shaquille Maitland-Smith was knifed in the stomach and sister Tahirah, 16, suffered serious slashes to her face.
Saturday's sickening attack in Hackney, East London, happened just half a mile away from where Ahmed Benyermak, 16, fell from a tower block.
One theory is that the latest killing may be linked to Ahmed's death 10 days earlier.
Shaquille, the 25th teenager to be murdered in London this year, and Tahirah are said to have been set upon by a gang on BMX bikes. Witnesses said the black youth's mum Sandra, 43, cradled him in her arms before paramedics arrived.
He died in hospital yesterday. Tahirah's condition was last night said to be stable.
Tiziano Niero, who lives four doors down from the dead boy's family in St Thomas's Place, said he saw a gang of youths riding BMXs and heard shouting outside his house before he was stabbed.
He said: "I heard a girl in tears and I looked out on to the street and there's a kid lying in the street, conscious but bleeding.
"He was dying in front of my window but dragged himself in front of his house.
"He was a nice boy. He wasn't aggressive but he got with the wrong crowd."
A police spokesman said: "It is believed that the two victims had been involved in an altercation with a group of youths prior to being assaulted."
Last night, Shaquille's godmother Cindy Liverpool, 46, told of his mother's grief.
She said: "She is not taking it well at all. It's so sad. He was just a really nice little boy. He didn't interfere with anyone."
Another terrified mum in the neighbourhood said: "It's not safe at all around here.
"When my children come home from school, I make sure they take the quickest route."