Final answer:
The sentence is rewritten with the comparative structure 'as..as' to be 'My sister and I are the same age, as we are both twelve years old.'
Step-by-step explanation:
The sentence "Both my sister and I are twelve years old." can be rewritten to use the comparison structure as..as by stating: "My sister and I are the same age, as we are both twelve years old." In this rewritten sentence, the as..as structure is used to convey that the speaker and the sister share the same age, thus expressing equality in their ages.