Questions & Answers about Её брат, мой дядя, моложе: ему сорок лет.
Брат is the grammatical subject of the sentence, so it must be in the nominative case: брат.
Её is a possessive pronoun meaning her (as in her brother). This word is indeclinable: it never changes its form for case, gender, or number. It is always её:
- её брат – her brother
- я видел её брата – I saw her brother
- я говорил с её братом – I spoke with her brother
Only the noun (брат, брата, братом…) changes for case, not её. In your sentence, we need nominative брат because we are stating who is younger: Её брат … моложе.