My sisters ___ a cake while my brother ___ in the park.

The sentence describes two simultaneous actions in the past. The first action, 'My sisters ___ a cake', involves a plural subject ('sisters'), requiring the plural past continuous verb 'were baking'. The second action, 'my brother ___ in the park', involves a singular subject ('brother') and also describes an ongoing action in the past, requiring the singular past continuous verb 'was jogging'. Therefore, the correct option is 'were baking / was jogging'.

Answer: were baking / was jogging