This is the 4126th most frequent Turkish word.
kardeş
Sibling.
Here, 'kardeş' is used with 'kız' to specify the feminine gender, referring to a sister.
Benim bir kız kardeşim var.
I have a sister.
In this sentence, 'kardeş' is modified by 'erkek' to denote a male sibling, meaning brother.
Ali'nin erkek kardeşi çok kitap okur.
Ali's brother reads many books.
The word 'kardeş' is used here without modification, indicating a general sibling, with the gender unspecified and understood from context.