Three friends — Aarav, Beena, Chetan — sit on a bench left to right

Why

Dev correctly used clue (1) to figure out that Chetan or Beena must be on the far left, since Aarav is ruled out — a solid first step. Where the reasoning stops too early is not checking whether clue (2) narrows this down any further. Clue (2) only rules out Beena from the far RIGHT — it says nothing at all about the far LEFT, so Beena is still just as free to sit on the far left as Chetan is. Test it: list every order that fits both clues — Chetan-Beena-Aarav fits (Aarav not leftmost ✓, Beena not rightmost ✓), and Beena-Chetan-Aarav also fits (Aarav not leftmost ✓, Beena not rightmost ✓, since Beena is leftmost here, not rightmost) — two different orders both satisfy every clue given. His sister is right: with only these two clues, more than one seating order is still possible — a clue that rules something out on one side does not automatically say anything about the other side.