Bros. Before Ho Ho Ho's is related to the seasonal world event, the Feast of Winter Veil. Completing this achievement is criteria for the meta achievement .
Though the list of brothers is much longer than the one for the Horde, they are very easy to get to. All the brothers in Stormwind City are close together in the Cathedral of Light. Goldshire is just outside Stormwind. Brother Nimetz is next to the flight point of the Rebel Camp in Stranglethorn Vale. Fly on to Booty Bay and take the boat to Ratchet. Fly to Theramore and find Brother Karman just outside the keep, where the fighters are training. Then fly to Nijel's Point for the final brother, who can be found at the Inn there.
If you do not yet have some of the flightpoints you should be able to travel to them even with a character as low as level 35. A determined player could even reach all targets with a death run at level 1.