Questions & Answers about Lei è estroversa, e lo è anche suo fratello.
Why do we use lo in the second half of the sentence?
Here, lo is a pro-predicative clitic. It acts as a placeholder for the adjective estroversa that was used in the first clause. While in English you can just drop the adjective and say "her brother is too," Italian grammar avoids leaving the verb essere hanging. It uses lo to effectively say "her brother is it too."
Since estroversa is feminine, why doesn't the pronoun change to la?
When used to replace an adjective or a general state of being, the pronoun lo is completely invariable. It never changes to match gender or number. Even though estroversa is feminine and fratello is masculine, lo stays the same because it replaces the abstract concept of the trait rather than a specific noun.
Why is the subject suo fratello at the very end of the sentence?
Italian often uses subject-verb inversion for emphasis. When adding a new subject that shares a previously mentioned trait, placing it at the end of the sentence puts the spotlight on that new piece of information. You could technically say e anche suo fratello lo è, but the inverted word order used here sounds much more natural.
As a quick review, why is there no definite article before suo fratello?