hawk-owl?
Hawks are daytime hunters; owls nighttime ones. Dawn and dusk you can find both out hunting, but dawn is more likely to be hawks, with the owls heading home to sleep, and the reverse at dusk.
If various predators are killing all your birds, you need to modify thier housing to protect them. It sounds as if they were much too young to be free-ranging, and/or that their fencing and coop/pen are not sufficiently secure.
If there really is an issue with your brother, speak to your...