The boy, who was 16 at the time of the attack, killed his 19-year-old sister in her apartment in Landskrona.
"This is an incredibly harsh sentence. My client is in shock," the teen's defence lawyer, Urban Jansson, told the TT news agency.
She was stabbed over 100 times with two different knives and a pair of scissors.
The boy had blamed the attack on a masked man, however the Lund District Court found the claims to be unsubstantiated and inconsistent.
The murder was "among the most serious cases of murder that can be committed" according to the court.
As the boy was only days away from his 17th birthday at the time of the attack, the judge decided to punish him as a 17-year-old rather than as a 16-year-old, altering the sentence therefore from four years to eight years in prison.
A psychiatric evaluation found no reason to place the teen in psychiatric care.
The sister had previously claimed she was forcibly married off at age 15 and raped.
In the months before her death, the 19-year-old had been speaking with the women's support group Tänk Om ('Think Again'), which works to combat honour violence. They had given her shelter and recorded her claims of abuse.