I am in no way a professional portrait photographer, and it was probably just dumb luck, but last year I was able to get a great shot of my son in a similar pose in front of the tree. I can post the image later, as I am on my iPad a and don't have access to that file, but the advice I can give you would be for you to move your girls away from the tree a few feet, so that the tree is a bit more out of focus. Also, step back a bit and get a bit more of the scene in there. As for the harshness of the flash, can you lower the power a bit, or bounce it off the wall or ceiling? Also, by moving them away from the tree a bit, the harshness of the flash might get a bit more diffused.....