the leg and foot mesh of SP3 is most likely stretched and morphed in the creation of this char, and the boots you are using expect to conform with the default mesh, so the boots cant logically conform any longer.

Your best option would be to make the heiress' feet and toes invisible, position the boots over the shins properly, parent the boots to SP3s shins (Boot>LShin to SP3>LShin and Boot RShin to SP3>RShin), then use the bend and twist dials on the boots feet and toes to get the proper pose with feet contacting the floor surface.