Neighbours and a passing group of youths in Hamburg's Sankt Georg district heard the girl screaming near the Berliner Tor metro stop and called police at 11:21 pm on Thursday.
The girl died about an hour later despite the efforts of emergency medical personnel to revive her, Hamburg police spokesman Andreas Schöpflin told The Local. Witnesses did not see anyone running away, he said.
Police have not ruled out the possibility that the stabbing could have been an honour killing, Schöpflin said. Homicide investigators and the district attorney's office in Hamburg are seeking the girl's 23-year-old brother.
"I cannot exclude the possibility of an honour killing, but I cannot confirm it either," Schöpflin said.
Both siblings are German citizens who immigrated from Afghanistan. Schöpflin could not say how long they had been in the country.
Schöpflin also declined to confirm a report from German newspaper Bild on Friday that the girl had been stabbed 20 times.
A crisis intervention team is caring for the girl's family.
A series of six honour killings in Berlin - including the shooting at a bus stop of 23-year-old Turkish woman Hatun Sürücü - shook Germany in 2005. Sürücü's youngest brother, Ayhan Sürücü, later confessed to killing her because he did not approve of her Western lifestyle.