According to your textbook, "Just as the frame of Vienna's tennis racket represents the structure of her family, the strings of her racket symbolize the ties that bind her family together" is an example of a

The statement is an example of a Transition

Explanation:

Transition is the literary device used when it is necessary for the author to use a word, or an expression, that connects two different thoughts expressed in the same sentence or in the same paragraph, allowing the text to have a pleasant fluidity and that the narrative is coherent and consistent.

In the passage shown in the question above, the expression "Just as" is used as a transition, because it allows the coherent connection between the two different ideas presented in the rest of the paragraph.