As women's rights groups rallied outside the regional court in the western city of Detmold, 22-year-old Osman Ozmen said he had kidnapped Arzu, 18, in November from the apartment of her German boyfriend, pulling her into his car.

They argued fiercely on a break from the drive and he said he pulled out a pistol he had brought with him.

"He indicated that his sister had provoked him and insulted him and his parents," a spokeswoman told AFP.

"He said he then lost control and shot and killed his sister."

Arzu's 27-year-old sister Sirin, brother Kirer, who is 25, and another sibling confessed in court to helping Osman kidnap her. Prosecutors said they were enraged by Arzu's relationship with the young German man.

Her body with two bullet wounds to the head was discovered near the northern city of Hamburg in January after a 10-week search that drew 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 group 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.