Slovenian is fairly close, and you can probably lump in Bulgarian and Macedonian in there as well. On one hand, I'd consider it a stretch for Bulgarian/Macedon

Slovenian is fairly close, and you can probably lump in Bulgarian and Macedonian in there as well. On one hand, I'd consider it a stretch for Bulgarian/Macedonian and Bosnian/Croatian/Montenegrin/Serbian to be considered the same language. On the other, there's also Torlakian (https://en.wikipedia.org/wiki/Torlakian_dialect https://en.wikipedia.org/wiki/Torlakian_dialect), a dying dialect that's been shared between Serbians/Bulgarians/Macedonians around the border. It's considered to be a dialect of both Serbian and Bulgarian, depending on who you ask, really bluring the line between the two.