The sixth sense wasn’t seeing dead people. It was seeing four dimensional space. Daria figured it out first, then she taught me. You just had to stare at something, like a chair, for long enough that when you closed your eyes you could still see it. Then with your eyes closed we could see into the chair, every level of wood grain, every puff of cotton in the cushion.
The seventh sense discovered was being able to “see” or “feel” the material/chemical properties of the stuff around us. We didn’t have words for it, so we called it “matting”.