During the sentencing, the court referred to the murder as an "honor killing," the Swedish news agency TT reported.
The teen's defense attorney, Urban Jansson, termed the sentence "incredibly harsh" and said his client was "in shock."
The 19-year-old woman was stabbed more than 100 times with two knives and a pair of scissors in April 2012 at her apartment in Landskrona.
The brother had said a masked man stabbed her.
Because the teen was only days away from his 17th birthday when the killing occurred, the judge sentenced him to eight years instead of the four years he would have received as a juvenile.
The sister had claimed she was forcibly married at 15 and raped. She had contacted a group that works to combat honor violence and that had given her shelter.