Minchandre,
I have difficulty understanding your last paragraph because OTL Serbs, Bosnians and Croatians are divided by religion. As the old empires extended their spheres of influence into the former Yugoslavia, they recruited local hill tribes to do the dirty work of fighting, smuggling, ethnic cleansing, etc.
Russia recruited Serbs and converted them to the Christian Eastern Orthodox faith. Meanwhile, the Holy Roman Empire (mostly Germanic) recruited Croatians and trained them to be good little Roman Catholics. Finally, the Ottoman Empire and converted Kosovars and Bosnians to suppress the other two tribes. Along the way Bosnians and Kosovars converted to Islam.
Many local villagers did not concern themselves with religious differences, so the ocaissonal Croatian family moved into a Bosnian village, etc. Few villages were religiously "pure" or tribally "pure." Serbians married Croatians, etc.
"National" boundaries are squiggly and largely ignored by locals before the former Yugoslavia collapsed into civil war as a handful of opportunistic politicians (e.g. Slobodan Milosovich decided that massacring neighbouring tribes was the best way to increase their personal power. One of Milosovich's biggest hassle was all those pesky Croatian families living in Serbian villages. Even worse were Bosnian villages within Serbian borders. If uncomfortable minorities like not be bullied into moving tithe "correct" side of the border, they were massacred.
After the last (Russian) Empire failed, hill tribes resumed their centuries-old feuds. Infighting during the 1990s was largely funded by smugglers and other organized criminal gangs.