Der Bruder eines/des neuen Nachbarn arbeitet bei Mercedes Benz.

 I am confused by this sentence from 10.8 on adjectives.
Der Bruder eines/des neuen Nachbarn arbeitet bei Mercedes Benz.

The new neighbor's brother works for Mercedes Benz.
My problem is "eines/des neuen Nachbarn"
Leaving aside that there is no hint that eines/des alternatives are required.
eines/des are both masculine genitive singular
Neuen could be various things including genitive singular or plural.
Nachbarn is plural  and I am not clear about genitive endings for nouns in the plural.

Why are eines and des singular and Nachbarn is plural?
 

der Nachbar/Nachbar
den Nachbar/Nachbarn
dem Nachbar/Nachbarn
des Nachbars/Nachbarn

What you see in the sentence is the weak declension.

Hope this helps!

Lucia