Proto-Slavic vьśi => Serbian svi, Russian vse. Proto-Slavic světъ => Serbian svet/svit, Russian svet. It's two unrelated words which just happen to start with

Proto-Slavic vьśi => Serbian svi, Russian vse. Proto-Slavic světъ => Serbian svet/svit, Russian svet. It's two unrelated words which just happen to start with the same two consonants in Serbo-Croatian out of all Slavic languages, after the phonetical change vs- => sv-