'Ukrainian and Belarusian are closer to Czech, Slovak and Polish than to Russian'
I have a Ukrainian friend who says this. it is clear that it is (at least partially) ideologically driven -- they are the type to habitually employ the word 'colonialism' at any given chance -- but I want to know the basis of this, considering that conventional linguistic grouping dictates Ukrainian, Belarusian and Russian are closest relating to each other than to those other ones, being in their own group.
apparently most of the research on this is in the Ukrainian language, which I don't speak nor write... so I am wondering if anyone else here has come across this notion or has anything to say about it?