>>281618206
Easy as then. Take a look at the bones moving in pose mode to see what parts they move and pick whichever one is closest to the thigh's movement. Reparent the 2 chains of bones from the part you want it to bend from in front of each thigh to their respective thigh. With the chain down the middle, use the vertex weight mix modifier to merge the same level of bone you changed the parents of into both legs then delete the bones you merged. You only need to reparent the bones in front of the skirt and can leave the back of the skirt untouched.
Extremely unhelpful pic related probably what you'll need to do.