、。
She'll grow up to be a fine pianist.
Literal
She [topic-は] adult [to-に] become-and excellent pianist [to-に] will-become probably.
Two ~になる ('become') links chained together by the te-form: 大人になって ('become an adult and') → 優れたピアニストになる ('become an excellent pianist'). 優れた is the past form of 優れる ('excel') used as a prenominal modifier — Japanese routinely lexicalizes verbs in their past form to describe a current state ('outstanding, excellent'). だろう at the end is the conjectural copula — 'probably / I'd guess.'