Blood doesn't count, culture is more important imo.
So what? Today`s Slavs are connected with both, blood and culture. Actually, we Europeans are all connected by blood considering all Whites originate from Slavs (ie Sarmatians) and even our culture isn`t that different but, see, language seams to be most important. Language is that key element of culture that define you as a being. After that, when you are aware of shared blood, it can strengthen the argument. We Slavs preserved that direct linguistic continuity with our ancestors and non-Slavic Whites didn`t. That is one of explanations why are Slavs hated by non-Slavs. One always hate what he was.
Does this include the Croats as well? ........... Bulgarians too.
It is as you ask do Orthodox Poles belongs in Polish and Slavic civilization. Of course, Croats are included but Croatia is society in great turmoil, where people are tear apart by the pressure of Vatican and Germany, not to be Serbs, while they know they are Serbs and they deny themselves and hate their former self -Serbian/Slavic (speaking of majority of populace). For Vatican its nothing personal. They doing it also in Bosnia, in Hercegovina, in Crna Gora, Dalmatia, Slavonia, etc our native regions. Its missionary work and they gave themselves right to do so, even if it mean to create new nations out of Serbian people. Its not easy situation.
Bulgarians? I don`t know what to say. Let`s say they more and more wants to be Slavs and they are part of Slavic civilization.