ISTANBUL — Two Kurdish German men accused of helping to kill their sister in 2005 because of her Western lifestyle were acquitted Tuesday in a Turkish court, in the latest failed attempt to prosecute them in a case that became a benchmark for cultural tensions between Turkey and Germany.

The sister, Hatun Surucu, 23, was killed at a Berlin bus stop when her youngest brother fired three bullets into her head. The brothers said the family’s honor had been offended because she divorced the man her family had forced her to marry at age 16 and then began dating and refused to wear a head scarf.

Though her family is ethnically Kurdish, and originally from Turkey, Ms. Surucu was born and raised in Germany. Her murder, after a series of similar so-called honor killings of Muslim women in Germany, sent shock waves through the country.

Some seized on the case to further their arguments that conservative Muslims are incompatible with secular German society, and it reflected more broadly on relations between Turkey and Germany at a time when some Europeans were arguing against allowing Turkey to join the European Union because of civil rights issues.

Ms. Surucu’s youngest brother, Ayhan, admitted that he had killed her, and he was jailed for nine years in a German prison.

But his brothers — Mutlu, now 38, and Alparslan, now 36 — have been acquitted twice of helping him: first in Germany in 2006, and again on Tuesday when they were found not guilty at a separate trial in Istanbul because of a lack of evidence.

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