The graphic below shows that all Slavs can understand each other to some degree, perhaps comparable to English-Dutch. They have moved away from the two Baltic languages. Lithuanian is regarded as the lest changed from PIE.
The East Slavic group diverged only a few centuries ago. Russian borrowed heavily from French, while Ukrainian shared with Polish. Langfocus on Youtube estimates 62% shared lexicon between Ukrainian and Russian, although the phonology may be 90% shared. The map below combines a map of Russian dialects (in Cyrillic) and those of Ukraine (Romanised). A continuous gradient can be seen from Arkhangelsk in the north to the Polish border. The dialect of Kyivan Rus (3 & 4, blue/ yellow) was dominant in the 9th century. The dialect of the Moscow area (7, mauve) become prestigious in high tsarist times and in the triumphant USSR in 1945.
There is a central question about national identity. Language is usually central, followed by religion, natural boundaries and sometimes a shared economy. “Englishness” is strongly associated with the sea border and the language, but we ignore the small peninsula of Anglia in northern Germany. The citizens of Donetsk and the Crimea are currently being persuaded to think of themselves as “ethnic Russians”. What does this mean?