Breakdown of Meine Schwester kommt mit ihrer Nichte.
kommen
to come
mein
my
die Schwester
the sister
ihr
her / their
Questions & Answers about Meine Schwester kommt mit ihrer Nichte.
Why does the word for "her" have an -er ending (ihrer)?
The preposition mit always forces the following noun phrase into the dative case. Because Nichte (niece) is a feminine noun, the possessive word ihr (her) has to take the feminine dative ending -er, becoming ihrer.
How do we know ihrer means "her" here, rather than "their"?
The base possessive word ihr can mean either "her" or "their" depending on the context. Here, it naturally refers back to the subject of the sentence, Meine Schwester. Since a sister is a singular female, we know it means "her".
If the subject was my brother, how would this affect the word ihrer?
You would need to change the root possessive word from ihr (her) to sein (his). Because the niece is still feminine and follows the dative preposition mit, you still add the -er ending. The sentence would become: Mein Bruder kommt mit seiner Nichte.
Does the noun Nichte get a special ending here because it is in the dative case?
No. Feminine nouns like Nichte do not change their spellings in the accusative or dative cases in the singular. All of the grammatical heavy lifting is done by the ending on the possessive word ihrer.