Whatever happened here, it was long ago, and it was deeply unpleasant.
With the door locked, inference quickly breeds an answer for Sulimann, something which he knows unfortunately well from his own upbringing: neglect can make death come fairly quickly, and so it did here. The two did not turn on one another, there's no evidence of fractures on bones so there was no abuse upon them. They were simply... forgotten, abandoned, left to die and die alone. No parents, no maids, no one to care but one another. So, desperate, they sat on the floor, elder sister holding younger brother, and the two merely fell into an unpleasant, unfortunate, and very hungry sleep from which they did not awaken, remaining forever embraced.
The room has little else in the way of surprises: the chest is full of toys, and the house is... a stunning replica of the house they currently stand in, Sulimann realizes. Four floors, with the third one exactly as they found it before, except more vivid in color than they saw, though this one to is covered in. Here on the attic level, the floors are the same, with two little dolls indicating the Rose and Bolt they met outside. The second floor seems to have a secret room hidden in the library, and the first floor has a hidden latch that leads... somewhere the house does not reveal, in the large room near the door from which they entered the house. There is another secret door on the attic level, too, in the one room they haven't checked, that leads down, spiraling down, across all floors, into the same hidden basement, but there is no indication about what lies beneath Durst Manor.
A small voice speaks over Sulimann's shoulder, as Dynaheir and Joliet see the two children manifest.
They look remarkably different, Rose and Bolt: they are darker, like specters, shades of themselves, and they look significantly skinnier and more sad.
"That one leads into the basement. That's where the Monster lives." Rose's voice is somewhat smug and cross, but distant, as if an echoing voice.
Bolt then speaks. "Rose, they're touching the toys! I don't like it!" he notes, as Dynaheir turns from the opened chest of toys.
"Mama says not to talk to strangers, especially peasants. Are you peasants? You are certainly strangers, and I shall call for her if need be. Now who are you, and what are you doing in our rooms?"
Joliet, Sulimann, and Dynaheir all get the distinct impression that this Rose speaks differently from the one before, and seems to genuinely not know who they are.