A 20-year-old US man who drugged and raped his 16-year-old sister has been handed a lighter sentence after a judge said the "stigma" of a sex conviction was enough of a deterrent.

Nolan Bruder pleaded guilty to giving his sister high-strength marijuana in the form of concentrated butane hash oil, known as dabs, after she repeatedly resisted his sexual advances.

The drug was dished it so she would no longer recognise him as her brother.

California Judge William H Follett sentenced Bruder to three years, but he will essentially serve only 120 days in a county jail.

He noted that Bruder's sister removed her clothes herself and was not unconscious at the time of the rape.

Judge Follett also questioned whether there was enough evidence for a jury to convict him, despite a filmed confession by Bruder.

Del Norte District Attorney Dale P Trigg said "he could not disagree more with" Judge Follett's decision, according to KRCR .

"The message that this sends to our community is that sexual predators who get their juvenile siblings stoned enough can have sex with them without any meaningful consequence," Mr Trigg said.

"That is not the message I want to send to our community.”

A mug shot photograph of Bruder

Mr Trigg claimed this case was "more egregious than Brock Turner," the 21-year-old Stanford University student who served three months of a six month prison sentence for sexually assaulting an unconscious woman, and called for a change in law.

"This defendant took advantage of a position of trust as this victim’s big brother. He knew she didn’t want to have sex with him. She told him that repeatedly," he said.

"So he got her stoned on dabs he gave her until she didn’t even recognise him in order get what he wanted."

Deputy District Attorney Annmarie Padilla has called on Judge Follett to follow theProbation Department's recommendation for six years in prison.

"Under this interpretation of the law, a perpetrator at a college party who chooses to forcibly rape a conscious victim will go to prison," she said, according to the Crescent City Times .