My daughter and her husband live in a house I own that is about 55 years old. Suddenly a few weeks ago they started noticing a tremendous stench all through the house when they use the shower on the third floor which dissipates after a while. There is also a full bath on the second floor and a half bath (no shower) on the first. The second floor is original to the house . The first floor was added in the 70's. The third floor was added in the late 80's/ early 90's. I haven't had time to investigate but they seem to think the smell is coming from the kitchen sink (first floor) every time they shower on the third floor. I just can't understand this. They have been living there for a year. What happened?

they seem to think the smell is coming from the kitchen sink (first floor) every time they shower on the third floor.
Do you hear bubbling or gurgles from the sink? What I think's happening here is the discharge from the shower is hitting a partial blockage in the main and sending a puff of sewer gas back through the sinks trap. The solution would be to snake the kitchen drain line from the kitchen roof vent. Put out enough cable to reach the base and 20 feet more to be sure. Flush the line out with a couple of sinks full of hot water. Good luck, Tom