Why Giulia's horror on the stairs?

Maybe there is something with the cat that was cut. My take, however-

True, she reacted coldly to the death of her mother, she's a very ambiguous character. When they're throwing her belongings out of the window, she's overjoyed to be ridding of what was cluttering their lives. Her mother, from her cold point of view, was the object keeping them from going further in their lives, keeping their brother from leaving them, and keeping them from growing (the mother asks an older boy if he wants candy when upset?). Also, the mother's statement before she's pushed off the edge seem to imply she knew what was coming, and that she wanted to die, and was glad to be assisted in her 'suicide'.

So, to the point, perhaps Giulia viewed her mother as someone who 'should' die, or believes that she wishes to die, but when it comes to the brother, she may view him as a more helpless victim due to his not being all there. It's easy for a cold individual to rationalize, but as the sister doesn't seem as detached as her brother, she may have accepted her mother's death, and believed all could be well following that. Her brother's death made things more "real" to her, and only in that moment of horror did she realize her brother's true brutality.

Maybe, anyway, best thoughts I have at the moment.

"Sir, you can't let him in here. He'll see everything! He'll see the big board!"

IMO, The sudden horror on the stairs was because she thought that Leone had simply fallen asleep (or possibly had an attack, remember that Ale threw a glass of water on the bathroom floor?) and drowned in the tub, but as she was climbing the stairs she realized that Ale did it. Also, I'm not totally convinced that Giulia believed Ale when he told her that he killed their mother.

Or you can call me KingMannerino, if you're not into that whole brevity thing.

Giulia wasn't seeing anything external when she looked up the stairs. No theoretically tortured cat is required to explain that expression.

She was perceiving that 1) Leon's death wasn't an accident, that Alesandro had done it, 2) his note before the drive wasn't just a prank, 3) he was dead serious about having killed their mother, which she might not have been absolutely sure of before, 4) even if the death of their blind ailing mother had benefits for them all, Ale was killing because he just plain liked it, and would even take out their totally harmless retarded brother, in which case 5) no one was safe--especially not her, if he became paranoid because she knew the secret.