。
My sister has long legs.
Literal
Younger-sister [topic-は] legs [subject-が] long.
The double-subject construction XはYが — 'as for X, Y is Z' — is how Japanese expresses inalienable attributes. 妹は (topic: my sister) + 足が (subject: legs) + 長い (long). English says 'she has long legs'; Japanese says 'regarding her, legs are long.'