The Swedish duo of First Aid Kit, comprised of sisters Johanna and Klara Söderberg, cite sibling acts like the Louvin Brothers, the Everly Brothers, the Roches, and, more recently, the Secret Sisters as influences, but those are only vaguely apparent on “The Lion’s Roar,” which has been a sleeper hit in indie-rock circles this year. Enough so that First Aid Kit is on a headlining tour, which stops at the Paradise Rock Club on Monday.