Her chest is exposed because the shirt doesn't fit over her chest properly like Minoriko's. Would this count as Wardrobe Error or Wardrobe Malfunction?

It would most likely be an malfunction but this looks nothing like Minoriko's outfit.

Take a second look. In the second version of the picture on pixiv the Aki sisters are in the background. She's not surrounded by autumn's leaves and a full harvest for nothing, either. It may be an adapted form but she's still wearing her outfit.