Quick one, I have heard a few interpretations of this and had some suggestions that there might be some cultural usage that isn't immediately obvious, hoping to get some clarity.
Please forgive the poor drawing, hopefully it is clear enough!
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It must have been written 義｛ぎ｝兄弟｛きょうだい｝.
Usually it means a brother-in-law. But, in this movie it means blood brothers: two or more men not related by birth who have sworn loyalty to each other. This is in modern times usually done in a ceremony, known as a blood oath, where having each person make a small cut, usually on a finger, hand or the forearm, and then the two cuts are pressed together and bound, the idea being that each person's blood now flows in the other participant's veins.