Terrell Davis, a 27-year-old rapper known as TM1way, was killed in Brixton while taking shopping to his grandmother, it's claimed.
He was pronounced dead at the scene.
London-born 21 Savage, whose real name is Shéyaa Bin Abraham-Joseph, and Terrell have the same father, Kevin Emmons.
The 28-year-old posted about the tragedy on Instagram on Monday night, writing: "Can't believe somebody took you baby bro I know I took my anger out on you I wish I could take that s**t back."
Kyra Davis, Terrell and 21 Savage's sister, also shared a heartbreaking tribute on Instagram tonight.
She wrote: "I don't even know what to say. My heart is in pieces.
"I love you more than words can say. Rest in perfect peace Terrell."
According to the MailOnline Terrrell got into an argument with an old friend, who stabbed him.
The source said 21 Savage will "be devastated, however, the whole family are.
"Terrell was a good guy, he used to live on the estate but had moved away."
A police spokesperson said: "Police are investigating a fatal stabbing after they were called at 17:59hrs on Sunday 22 November by the London Ambulance Service to a location in Ramillies Close, Lambeth, SW2.
"Officers attended, along with colleagues from the London Air Ambulance, but the 27-year-old man, was pronounced dead at the scene.
"The man's next of kin have been informed and they are being supported by specially trained officers.
"No arrests have been made at this stage. A crime scene remains in place at the location, and officers are continuing their enquires."