Honestly, if this wiring is from the 1950s and every handyman on earth has emssed around with it ever since, convince your parents of a full rewire! Anything else is a band-aid approach, and now is th best time to do this!
I'm in a similar situation right now, I live in a small family-owned apartment builidng (early 20th century) with rather shady wiring in some areas. After a fire on the top floor about a week ago (probably not caused by faulty wiring but rather a faulty fan heater that set a wastepaper basket on fire across a remarkable distance) I'm on a mission to convince everyone of a thorough cvheck-up of the existing wiring. I definitely know that there is some original 1915 wiring still in use and I don't like that! The original sparks weren't easily scared - they only used conduit where wirepenetrated plaster & lath ceilings or in plastered masonry walls, inside the ceilings individual cloth-covered wires rest loosely on top of the wood lath! I disconnected one of these traps after the entire ceiling got drenched with a few hundred gallons of water, but I'm sure some remain. Unfortunately, my uncle seems to assume that anything that works is safe.
