兄弟 is a formal way of saying 弟弟and哥哥 but 兄弟 doesn't just mean your elder or little brother it also means your good friends and buddies,who have no biological relations with you ex:Tom是我的好兄弟. In this sentence,tom is your close friend.

Original meaning and usage of 兄弟 is indeed "brother", but in spoken language it can also refer to "pal", "mate", or "homie", etc. Example: 兄弟們，一起奮鬥拿下冠軍吧! Guys, let's be the champion this time!

No,兄弟 not just means 弟弟 这是我的兄弟。 It may has three meanings ,so which meaning is right that need to be depend on specific situation. First meaning :he is my little brother Second meaning: he is my bigger bother. Third meaning: he is my good friend.

From my understanding, "兄" is for elder brother and "弟" is for younger brother. So, basically "兄弟" is "brothers" (something more general), not an informal way of saying "弟弟". For example: 我们是兄弟 = We're brothers. 他是我的兄弟 = He's my brother. 他是我的弟弟 = He's my younger brother. Hope it's not confusing.. ><

So, you are right. It sometimes, as in your sentence(though it would sound more natural if you add "我" before "兄弟), means a YOUNGER brother, in which case the "第" has neutral tone di rather than 4th tone dì.