My first thought was that they were wired in series, but I know nothing about US wiring systems and regulations. Going by your descriptions, if I was to do an inspection and test to UK regulations (part of my day job BTW), I would have no hesitation in condemning your wiring and insisting on at least a partial re-wire. I'm not trying to worry you, just pointing out the differences.
MACMC seems to know what he's talking about, so between him and your father-in-law you'll probably get it sorted IMO. If not, call in a sparky
