Final answer:
The correct verb form is 'dice'.
Step-by-step explanation:
The correct verb form to complete the sentence is dice.
In this sentence, the subject is 'mi hermana' (my sister) and it is singular. The verb form that matches with a singular subject is 'dice', which means 'says' in English.
Example: Mi hermana dice la verdad. (My sister says the truth.)
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