(Revision #4) England 1963. When two teenage sisters, sibling rivals, secure a recording contract that launches their singing career, they soon discover that the Rock?n Roll lifestyle is full of seductive excesses that may destroy their dream of fame and fortune.

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.

This doesn't describe a story, rather a situation. A story needs a plot, and a plot needs an inciting event that motivates the main character to take action.

What is the inciting event? Is it securing a recording contract? Is it discovering the Rock n' Roll lifestyle? If so, what does that mean in specific terms? Do they start taking drugs, drinking, sleeping around and contracting an STD?

Whatever it is, the event has to be a single out of the ordinary incident, which motivates them to achieve a goal. As a logline needs to demonstrate a cause and effect relationship between the inciting event and goal, the goal can't be random or unrelated.

Also, it's hard enough writing a story about one protagonist, let alone two. therefore, it's better for early-career writers to write a single protagonist story. Pick one of the two sisters, and focus the story around her.

Lastly, there have been many stories about the rise and fall of a musician/singer - some fictional some based on real people. What is it about this idea that makes it different to the plethora of other very similar stories?