They're Orthodox and they're Slavs. But throughout the Cold War they were enemies. Yugoslavia liberated itself with little help from the Soviets, refused to join the Warsaw Pact (unlike every other Communist country in Europe) and were expelled from the Cominform when they stood up to Stalin.
Russia. Soviet Union. Serbia. Yugoslavia. Mihailovich. Tito. Stalin. Broz. Dzugashvili. Brezhnev. Putin. There’s enough going on for any generalization to need a few clauses added.