Putting thighs in is certainly a good first step. Now when you pose the figure move the thighs not the buttocks, which is what I think you have done in your image. From there you can add more ghost parts as the others have said, just remember that the more complex the more chances it will go wrong. Simplest from what you have now is to add some morphs to the lower portion of the robe. Flare front, flare back etc.

For greater refinement see what I did in my beachwear set. There the whole skirt is the hip group. The figure's hierarchy includes buttocks, thighs and shins parented to match Vicki. Then to each of the lower body parts I have parented an extra group run by JCJ coding. It is these extra parts that control the hip of the skirt.