What a fascinating and challenging puzzle-platformer. The game's core mechanic is very easy to understand, yet actually executing it is a significant mental challenge. I felt a similar mind-stretching sensation to when playing Portal for the first time. The visuals are spartan, yet somehow match eerily well with the ominous, weighty sound design and haunting music, which prepped me suitably for the macabre ending. I especially like the audiovisual treatment of the bond between the siblings--the line changing color as it stretches is inspired.
My only bit of constructive criticism is that I still found the controls a little obtrusive long after I had figured out the puzzles, though upon reflection I would argue that taking my own skill into account is an important part of solving these puzzles. That is: "Now that I've determined a way to pull this section off, can I actually do it? is there a way to make it easier to execute?" Overall, this is a thoughtfully and beautifully designed game. Great work!