My working theory is that they're all plants, except maybe for the masked man who might be a gardener. Possibly, onii-chan is a succulent and water comes from his arm, though that section of him can be splinted back on just fine because, he's a plant.
...
Honestly, this might be one of my favorite from the "100th chapter" series so far, in that it (unlike most of the others) takes the prompt seriously and doesn't explain a goddamn thing about what the flying hell is going on, and instead decides to roll with that, take it up to eleven, and make things weird as all get-out.