Girl with the broken wing (2017 reissue)

The twins are in bed when it happens: they hear a sound like a wet cabbage hitting the wall, a clattering on the roof tiles, then a tapping at the window. It's a girl with a broken wing - and she's come to stay. Who is she? Where has she come from? And more importantly, what does she want? Perhaps she's their guardian angel ... but would an angel snore and have filthy feet?