YOU SAID:

my sister is my older brother's younger sister and my older sister.

INTO JAPANESE

私の妹は私の兄の妹と私の姉です。

BACK INTO ENGLISH

My sister is my older sister and my older sister.

INTO JAPANESE

私の妹は私の年上の妹と私の姉です。

BACK INTO ENGLISH

My sister is my older sister and my older sister.