In its judgment, the court writes that that there is "convincing evidence" that the boy shot and killed his sister. The boy will now undergo a psychiatric examination before the court decides whether he should face a custodial sentence for the crime.
The boy has continually denied killing his sister, who was found shot to death on the family's farm in early August.
But prosecutor Bengt Åsbäck wasn't surprised by the verdict.
“It's a hard case to try, so it wasn't a self-evident, but I would have been surprised if the decision had gone in the other direction,” he told the TT news agency.
The media attention surrounding the trial exceeded anything Åsbäck has ever experienced, who said he hopes the justices succeeded in blocking it out and only focusing on what was said in court.
“There can be some concern about it. You don't know where the court gets its information until the end, when things start to circulate,” said Åsbäck.
“What's important is that what's said in court should be the basis for the ruling and nothing else.”
As the trial came to a close on Friday, the prosecutor in the case noted that there were signs that the boy, who suffers from ADHD, had seen his mental health deteriorate over the course of the summer.
He also explained that people with similar conditions are over-represented among convicted criminals, and that the boy's 19-year-sister had grown increasingly anxious about her little brother.
According to Åsbäck, the sudden change signified that the boy likely suffered from some sort of mental illness.
He also reviewed the forensic evidence in the case, including traces of the victim's blood in her brother's room, marks on a bloody bundle which was also tied to his room, the boy's story about the bundle, and finding his underwear with traces of his sister's blood in the room where the murder took place.
Attorney Jörgen Malmunger, who defended the 15-year-old, countered that his client, an impulsive teen with ADHD, wasn't capable of committing such a cold-blooded killing.
In the end, however, the court agreed with the prosecution, concluding that the boy had indeed murdered his 19-year-old sister.