The word brother is used in two ways in Bukhari. The first meaning is an actual blood brother, sharing the same mother or father. The second meaning is another Muslim.
This work is important since it means that pseudo-Golden Rule hadiths that say a Muslim is to love his brother does not apply to kafirs. Hence, the hadith is not a true Golden Rule, which is to treat ALL others as a brother. The kafir is outside the pseudo-Golden Rule. In Islam the Golden Rule is restricted to other Muslims.