I think as it’s written now, the fact that they’re “sibling rivals” doesn’t have to be in the logline.? It’s also ambiguous about whether they secure a recording contract as a duet, or if they both happen to get recording contracts at the same time.? I’m assuming duet.? I think the logline should focus on the rivalry, and that would probably let it more clearly describe a story.

If I were to watch a movie with the logline you wrote, I’d expect it to be pretty close to one of those movies where the band gets a recording contract, then falls apart because the lead singer starts partying too much.? The singer says, “I don’t need you guys!? I’m going off on my own!” and then eventually comes crawling back, asking for forgiveness.? I think if it’s about just two people, and they’re rivals before they even got a contract, that would be something more interesting.? If the logline convinced me that the fact that they were sibling rivals would make a difference in the story, I’d probably want to watch it.