der Bruder
brother
A male person who has the same parents or one parent in common with another person.
Mein jüngerer Bruder und ich verstehen uns sehr gut.
My younger brother and I get along very well.

brother
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brother
A male person who has the same parents or one parent in common with another person.
Mein jüngerer Bruder und ich verstehen uns sehr gut.
My younger brother and I get along very well.

The adjective 'brüderlich' (brotherly) and the noun 'die Bruderschaft' (brotherhood) are derived from the noun 'der Bruder' (brother). All these words share the idea of close, loyal connection.
Sie teilten das Essen in brüderlicher Eintracht.
They shared the food in brotherly harmony.
To specify age, you use 'älterer Bruder' (older brother) or 'jüngerer Bruder' (younger brother). In everyday language, 'großer Bruder' (big brother) and 'kleiner Bruder' (little brother) are also very common, often describing the social role as well.
Mein großer Bruder hat mir das Fahrradfahren beigebracht.
My big brother taught me how to ride a bike.
Using 'Bruder' (brother) to address a friend is very colloquial. It is used almost exclusively among young men to express a close, buddy-like relationship.
Was geht, Bruder? Lange nicht gesehen!
What's up, brother? Long time no see!
The religious meaning is usually clear from the context. Either the word is used as a title before a name ('Bruder Anton' / Brother Anton), or people are talking about a monastery or an order.
Die Mönche nennen sich untereinander Bruder.
The monks call each other brother.
friend / boyfriend
'der Freund' and 'der Bruder' can both refer to a close male person, but normally they mean different things. 'Der Freund' is someone with whom you have a personal, friendly relationship. 'Der Bruder' is usually a family member, meaning a son of the same parents or of one shared parent. Sometimes 'Bruder' is also used figuratively for a very close friend or in a group that wants to show strong solidarity. This can confuse learners. In everyday language, however, the important point is: a friend is not automatically family; a brother is normally family.

Kommt dein Bruder nach dem Unterricht auch mit dem Zug?
Is your brother also coming by train after class?
Mein Freund hilft mir morgen beim Umzug, obwohl wir nicht verwandt sind.
My friend is helping me move tomorrow, even though we are not related.