I think if this were a mother and child instead of siblings it would work better. Something like "Keeping fit with Baby!" It's a nice portrait, though the white balance seems a bit off to me. Also, the line going from her wrist to the edge is very distracting and I wish her whole foot were in the shot.
The white space at the top could be cropped makeing the presentation much more effective. Rich detail and evokes a sense of well being and health. Not a ribbon contender, but certainly a top ten image.
Would have been more stock like without the skirting board being visible and if you had put a white sheet down for them to sit on.
Nicd clear crisp image though with very good DOF