Blood Idioms (American idioms) 美式慣用語

blood brothers 
親兄弟;情同手足之朋友

explanation : This can refer to the actual birth brother, but it's also an expression to indicate two friends who are extremely close. In myth and legend, close friends often swear an oath of "blood brotherhood"

e.g.  Ever since Tom and Rufus went hunting together, they've done everything as a team.  They're like blood brothers!