Der Bruder eines/des neuen Nachbarn arbeitet bei 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?