Going from Sinhala to Javanese (Basa Jawa) has a composite linguistic distance of 83% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Sinhala → Latin/Javanese
Indo-European ↔ Austronesian
The 5 languages with the lowest linguistic distance from Sinhala:
A quick 3-question check between Sinhala and Javanese. No signup.