|A pandemic among humans would kill millions worldwide|
Her brother, 14-year-old Mehmet Ali Kocyigit, had already died of the H5N1 strain of bird flu, officials said on Wednesday, confirming the first human death from the disease outside China and southeast Asia.
"We lost Fatma Kocyigit this morning," Niyazi Tanilir, governor in the eastern province of Van, said on the CNN Turk news channel. Newspapers said Fatma was 15 years old. She died at around 6:30am (0430 GMT) on Thursday.
The governor said one patient was in a critical condition and another in a less serious condition.
A World Health Organisation (WHO) official said the boy had probably died from H5N1, which would mark a dramatic shift westwards for the disease.
Recep Akdag, the Turkish health minister, gave no specific details about the boy's death, but said samples had been sent to the WHO and Britain for more tests.
If the boy's death is officially confirmed as being the result of H5N1, it would be the first outside eastern Asia where more than 70 people have been killed by the disease since 2003.
The virus remains hard for people to catch, but there are fears that it could mutate into a form easily transmitted among humans. Experts say a pandemic among humans could kill millions around the globe and cause massive economic losses.