Going from Sinhala to Basque (Euskara) has a composite linguistic distance of 82% (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
Indo-European ↔ Isolate
The 5 languages with the lowest linguistic distance from Sinhala:
A quick 3-question check between Sinhala and Basque. No signup.