As women's rights groups rallied outside the regional court in the western city of Detmold, 22-year-old Osman Ozmen said he had "lost control" and shot his sister Arzu, 18, in November, according to the report.

Her sister Sirin, 27, and brother Kirer, 25, confessed in court to helping kidnap her. Prosecutors said they were enraged by Arzu's relationship with a German man.

Her body with bullet wounds to the head was discovered near the northern city of Hamburg in January after a 10-week search that received intense media attention, with photographs of the woman with cropped blonde hair splashed across newspapers.

The family originally from eastern Turkey is Yazidi, a highly traditional Kurdish ethno-religious group.

Prosecutors say her siblings, ranging in age from 21 to 27, could not accept her seeing a man from outside their community, which numbers about 60,000 in Germany.

Three of the defendants are on trial for murder and all five are charged with kidnapping.

Five hearings have been scheduled during the trial with 30 witnesses called to testify. The clan's father, who is the subject of a separate investigation, is not expected to address the court during the siblings' trial.

Rights organisations held a vigil urging the German justice system not to view the family's religious beliefs as a mitigating factor and to urge better protection for potential victims of "honour killings".

There are no official figures available on "honour killings" in Germany.

A Berlin-based organisation that seeks to help young women and girls threatened with violence or forced marriage, Papatya, has said 45 known cases occurred in the country from 1996 to 2004.