Look at how close they are!
As you can see, once you start getting over 90% cognates (see Ukrainian-Belorussian and Czech-Slovak) you are very close to the same languages. At the very least, you have two very closely related languages, and I do believe that Belorussian is separate from Ukrainian and Czech is separate from Slovak and MI tests show this to be true (MI 82% between Czech and Slovak).
Note that MI is apparently lower between Belorussian and Russian than between Belorussian and Ukrainian. That is interesting because many Russian nationalists say that Belorussian is a Russian dialect. Note also that Bulgarian and Macedonian, often said to be one language, have fewer cognates than between Czech and Slovak. Based on this chart, Bulgarian and Macedonian surely appear to be separate languages, as far apart as Polish and Slovak.
Note how close both Czech and Slovak are to Polish and Slovenian! Note also how close Serbian is to Slovenian and Macedonian.
It is also interesting how close Upper and Lower Sorbian are to each other. I wonder what the MI is like between them.
It is really amazing how closely related the Slavic languages are to each other.
