、。
She's a pianist, and her younger sister is a singer.
Literal
She [topic-は] pianist is-and-, younger-sister [topic-は] singer is.
The connector で here is the te-form of the copula です — used to chain noun-predicates together: 'is X, is Y.' This pattern is one of the cleanest ways to introduce two pieces of information about a topic in parallel: 学生で、二十歳です ('I'm a student, twenty years old'). 妹 ('younger sister') reflects Japanese's age-rank distinctions in family vocabulary — 姉 / 妹 / 兄 / 弟 mark older or younger explicitly, while English just says 'sister' or 'brother.' Note the kana spelling 妹 is used directly without an honorific, because she's a member of the in-group (家族, uchi).