The genetic relatedness of Ukrainian and Russian within the East Slavic branch of the Indo-European language family is a matter of established linguistic consensus and common knowledge, requiring no specific citation.
The claim states that Ukrainian and Russian share high lexical and grammatical similarity due to common Slavic roots. This is a matter of linguistic classification and common knowledge within historical linguistics (both are East Slavic languages sharing a common ancestor), making formal citation unnecessary under Step 0 rules.