兄长 a respectful form of address for an elder brother or a man friend
兄弟 兄弟[口语]1.(弟弟) younger brother2.(男子跟辈分相同的人或对众人说话时的谦称) your humble servant; i3.(称呼年纪比自己小的男子) a familiar form of address for a man younger than oneself 大兄弟, 这件事就托你了。 and so, brother, i'll leave the matter in your hands.