der Bruder

Der Bruder (the brother) is masculine and takes der. The plural is die Brüder, built with an umlaut plus an -er ending. That combination makes it look quite different from the singular, so the plural is worth learning as a separate word rather than as a small change.

To talk about brothers and sisters together, German uses die Geschwister (siblings), a plural only word. Ich habe einen Bruder und eine Schwester (I have one brother and one sister) can be shortened to ich habe zwei Geschwister.

Compounds follow the usual family pattern. Der Halbbruder is a half brother, der Schwager is a brother-in-law (a separate word, not a compound of Bruder), and der Zwillingsbruder is a twin brother. Die Bruderschaft means brotherhood or fraternity, and the adjective brüderlich means brotherly.

One phrase you will hear in older texts and songs is Brüderlein (little brother), a diminutive built with -lein. Diminutives in German always turn neuter, so it becomes das Brüderlein even though Bruder is masculine. The same rule gives you das Schwesterchen from die Schwester.

Pronunciation: Bruder is [ˈbruːdɐ], two syllables with the stress on the first. The u is long, like the "oo" in English "food". In the plural Brüder, that vowel becomes ü: shape your lips as if for "oo" but say "ee". Getting this vowel right is what makes the singular and plural sound clearly different.