Here's how I see it, and why:

Jack arranges the bodies and covers them.

When his father dares him to open the door, he eventually does open it. At this point his father does not intend to kill him, rather he wants to destroy him first, and lead him to commit suicide; being a monster he is, he takes pleasure in seeing Jack overwhelmed with grief and guilt.

Jack is fully aware that his siblings are dead when he attempts suicide. Then he sees them, and note the way he finds them sleeping: they mirror the arrangement of their bodies in the attic. Father is still up there. The door is open (there's no screaming - he's not locked up,) but he is in no hurry to leave the scene of his crime and feeling safe and content, waits to hear the shot that would end his family. Instead, Jack walks upstairs and locks, and then bricks up the door...

Sasha

Here's how I see it, and why:

Jack arranges the bodies and covers them.

When his father dares him to open the door, he eventually does open it. At this point his father does not intend to kill him, rather he wants to destroy him first, and lead him to commit suicide; being a monster he is, he takes pleasure in seeing Jack overwhelmed with grief and guilt.

Jack is fully aware that his siblings are dead when he attempts suicide. Then he sees them, and note the way he finds them sleeping: they mirror the arrangement of their bodies in the attic. Father is still up there. The door is open (there's no screaming - he's not locked up,) but he is in no hurry to leave the scene of his crime and feeling safe and content, waits to hear the shot that would end his family. Instead, Jack walks upstairs and locks, and then bricks up the door...