Rereading this now I realized, that the reason that Yuki was leaving that teddy bear in the school was because she know that Yurii really miss her sister and by the time Yuki left the teddy bear, Yurii started to hallucinate that it was Ruu, and Yuki seeing Yurii holding that teddy bear, Yuki is basically forced to play along and act like it was actually Ruu-Chan.