Belarusian and Ukrainian were connected with each other for a longer time, Russian went own way earlier. I refer to the Ruthenian language here. There's similiar relation between Ukrainian and Belarusian as there is between Slovak and Czech. Polish language was more connected with languages spoken in present-day East Germany. There are groups within East and West Slavic family, languages are not the direct "subdivision" of these families.
Currently Polish is most related to Kashubian language and probably Sorbian. Other languages are unfortunately extinct.
Can't say much about South Slavic languages, although I can understand something when people use it to communicate here. Mostly something like "sranje" (which in Polish means something else), "jebem ti majku" etc. you know - Aprician way of talking.
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