You're getting two classes of response here, so it might help to distinguish them. To make a robe into a conforming garment, it needs a hip and thighs, but need not have shins, as it won't move with them. The hip section should reach to the bottom between the thigh sections both back and front unless the front is open. It pays to make the bottom end wide enough for the legs not to pass through when the character walks. If you want some part of the robe to be movable independent of the wearer, then you need extra sections that do not have the same part names as the body parts, so that there will be no attempt at a conforming movement. Some of these may be better done with morphs, anyhow.