своей сестры

one's own sister

→ свой
phrase Rare #44,108
one's own sister Used when the sister belongs to the subject of the sentence. Often contrasts with его/её when the possessor is the same person as the subject.
RU: Он очень любит своей сестры.
EN: He loves his sister very much.

Том сделал бы всё для своей сестры Мэри.

Tom would do anything for his sister Mary.

Том гораздо старше своей сестры.

Tom is much older than his sister.