The Salten District Court ruled that the 25-year-old man spiked two cans of soft drink with a cocktail of amphetamine, methamphetamine and diazepam before he abused her.
"It is found that the defendant grossly exploited the victim's trust in him as her brother," the court ruled. "He took advantage of the victim's trusting relationship to satisfy his need for sex."
According to the court, the man had long been encouraging the girl to skip school, and had frequently offered her cannabis.
When he offered her the drug-filled soda, the court heard, he explained the strange taste by telling her he had put vitamin powder in it.
The man claimed in court that his half-sister had known that the drink had been spiked, contradicting a statement he made to the police immediately after he was arrested.