My sister ____ swimming every Sunday. (to go)

The sentence describes a habitual action that occurs every Sunday. This requires the present simple tense. Since 'My sister' is a third-person singular subject, the verb 'to go' needs to be conjugated accordingly. The correct conjugation for the third-person singular in the present simple tense is 'goes'. Therefore, the complete sentence is 'My sister goes swimming every Sunday.'

Answer: goes