Questions & Answers about Её брат, мой дядя, моложе: ему сорок лет.
Why is it её брат and not something like её брата?
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.
What is the difference between её and она?
Она = she. It is a subject pronoun, used as the subject of a sentence:
- Она моложе. – She is younger.
Её = her. In this sentence it is a possessive pronoun (her brother). It can also be the object pronoun her:
- Я люблю её. – I love her.
- её брат – her brother.
So you use она when she is the one doing or being something, and её when you mean her as a possessor (her X) or an object (I see her).