Hmmm I wouldn't call them "brothers" in any way possible. I think that they also have a negative view of each other, both have greater hate for the third side. For example:

1. Albania-Croatia: common hate for Serbia and the Serbian nation, both most people in Croatia proper have a general dislike for Albanians (mostly from those who hail from Kosovo). Albanians who are predominantly Muslim with minority of Christian Roman Catholic and Orthodox dislike Bosnian Muslims (especially from the Sandžak region in Serbia and Montenegro) with the Croats also hating Bosnian Muslims. As long as common aversion to Serbia exists this makes them common allies, both not brothers in any way.

2. Serbia-Greece: common hate for Albanian and Turkish nations, as well as it's people! Both are fellow Christian Orthodox nations with a history of fighting a common enemy through nineteenth and twentieth century, both the Hellen's have a great dislike of neighboring Slavic nations, especially against Bulgarians and Macedonians whom they constantly point as the main culprits who have taken their history and reduced their territory to what it is today. Serbs also dislike Bulgarians very much, both have mixed feeling when it comes to Macedonians and it's statehood. Quite an exotic pair of allies if you ask me, both I think that the two would clash the moment their border come in contact, let's hope that scenario never comes.

3. The Turkish people generally have a dislike of Balkan nations, and considered themself superior in every way when comparing themselves to this part of the world. The Bosnian Muslims who live in Bosnia & Herzegovina are split between those who have positive views of Turkey, while others see them as occupiers who forcibly converted to Islam and eradicated their autochthonous "church of Bosnia". The Bosnian Muslims generally hate the Croats as well as the Serbs, the Turks generally have good relations with Serbia on the economic side, similar political systems and are welcome guest in Belgrade, be they tourists or foreign workers looking for work. Not to forget that Bosnia & Herzegovina has given asylum to members of the "Gülen movement" who are currently in conflict with the Turkish regime. These are dysfunctional allies who are split how to act than they are to come to an agreement is going to take charge

In the Balkans there really is no middle ground, it simply can't be because these people think more with their hearts than their brains...and that is going to stay that way until the end of times comes!