Going from Sicilian to Croatian/Serbian (Hrvatski) has a composite linguistic distance of 67% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Latin → Latin/Cyrillic
Indo-European ↔ Indo-European
The 5 languages with the lowest linguistic distance from Sicilian:
A quick 3-question check between Sicilian and Croatian/Serbian. No signup.