The noun 兄弟 (xiōngdì) means "brothers" or "siblings" and can refer to biological brothers or be used more broadly to mean close male friends or comrades. It is a common word in both everyday conversation and literary contexts.
他们兄弟俩从小感情就特别好。
Tāmen xiōngdì liǎ cóng xiǎo gǎnqíng jiù tèbié hǎo.
The two brothers have been very close since they were little.
我和他是多年的好兄弟,什么都可以说。
Wǒ hé tā shì duō nián de hǎo xiōngdì, shénme dōu kěyǐ shuō.
He and I have been good brothers for many years; we can talk about anything.
三兄弟一起开了一家餐厅,生意很红火。
Sān xiōngdì yīqǐ kāi le yī jiā cāntīng, shēngyì hěn hónghuǒ.
The three brothers opened a restaurant together, and business is booming.