A writer’s life has turned upside down when he suspects his mentally ill twin brother who kidnapped, sexually abused and murdered his daughter.

So what’s he going to do about it? You’ve given us an inciting incident, but that’s all.

“life has turned upside down” – This is a pet peeve of mine in a logline. All it means is (when your really think about it) something dramatic changes in the protagonist’s life. That’s it! And that is kinda one of the fundamentals of storytelling. Something has happened that is worth writing a story about. In my opinion, it’s pretty much a given.

Your hero is simply a “man”. This guy is supporting the weight of the entire story on his shoulders so perhaps consider fleshing him out a bit more. “Man” could be anything from 18-100+. In story terms, that changes things considerably. If his daughter died 40 years ago, it would be a very different story to if his daughter was found dead 2 days ago. So he’s a man… but who is he really?

“Double Suspicion” – According to your logline, he doesn’t suspect his twin brother, he discovered this to be true. So there’s no suspicion at all.

Since you’ve put in this in Thriller, my guess is that he doesn’t just go to the police. If it was a story about dealing emotionally with the consequences of this event, I imagine it would be in Drama. Since it’s Thriller, my guess is that he’s going after his brother. You need to tell us that! You need to tell us what this “man” is going to do about this. That’s key. Without a goal, you’ve got nothing for the protagonist to do after Act I.

Hope this helps.