to his/her own sister
Я хочу купить своей сестре рубашку.
I want to buy my sister a shirt.
I gave my sister a dictionary.
I bought this for my sister.
Он представил меня своей сестре.
He introduced me to his sister.
Том одолжил его своей сестре.
Tom lent it to his sister.
Том напишет своей сестре.
Tom will write to his sister.
He called his sister.
Call your sister.
Она представила меня своей сестре.
She introduced me to her sister.