Raj says, "I am my father's only son." Kunal thinks this does not decide whether Raj has a sister…

Why

This one is actually a matched pair, not a disagreement — Kunal and his friend have both landed on exactly the same correct conclusion, even while sounding like they might be arguing. 'Only son' is a claim specifically about SONS — it says Raj's father has no other son besides Raj — but it says nothing at all, one way or the other, about daughters. A family could have exactly one son (Raj) and zero daughters, or exactly one son and two daughters, or any number of daughters — 'only son' stays equally true in every one of those cases. Test it: try stating the opposite claim to see what it would actually rule out — 'Raj is NOT his father's only son' would mean there is at least one other son, but even that says nothing about daughters either. Both are correctly saying the same thing: 'only son' tells you about sons alone — whether a sister exists is genuinely undecided information, not something the sentence quietly implies either way.