
der Neffe
nephew
The son of one's brother or sister.
Mein Neffe aus Hamburg kommt uns am Wochenende besuchen.
My nephew from Hamburg is coming to visit us this weekend.
nephew
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nephew
The son of one's brother or sister.
Mein Neffe aus Hamburg kommt uns am Wochenende besuchen.
My nephew from Hamburg is coming to visit us this weekend.
'Neffe' (nephew) is a weak masculine noun (N-declension). This means it takes the ending '-n' in all cases except the nominative singular. This also applies to all plural forms.
Ich schreibe meinem Neffen eine E-Mail.
I am writing my nephew an email.
The word 'Neffe' (nephew) is almost always used with a possessive pronoun (mein, dein, sein, ihr, etc.) to clearly express the family relationship.
Sein Neffe hat gerade die Schule abgeschlossen.
His nephew has just finished school.
To clarify the exact family relationship when it is not clear from context, you can use the structure 'der Neffe von + dative' (the nephew of + dative).
Das ist Peter, der Neffe von meiner Kollegin.
This is Peter, my colleague's nephew.
niece
Neffe and Nichte both refer to the child of a brother or sister. Neffe is used for a son, and Nichte for a daughter. What matters, therefore, is the child's gender, not the gender of the sibling.

Mein Neffe besucht mich am Samstag mit seiner Mutter.
My nephew is visiting me on Saturday with his mother.
Meine Nichte besucht mich am Samstag mit ihrem Vater.
My niece is visiting me on Saturday with her father.