Amagoi

From MangaHelpers:

The story begins with regret, chaos, and desperation. Sora lives with his little sister, An, and his rather comical father. An is the soloist of her school's chorus, and Sora is an awesome cook. Sora prepares his "honki" (serious) bento for his sister's school concert.

Once An begins her solo, a mysterious force creates a rift in reality, as An's existense shatters right before everyone's eyes. Sora runs in to save her, but he too becomes part of this dark ritual. As Sora ends up transported to a fantasy world full of gigantic mushrooms, nothing can be certain on what will happen to these two.