Is there good ventilation in the rooms that smell? I assume you checked out under the sink for dampness. Take a long artist type brush(just get a cheap one) or thin wooden dowel & put a thin clean white cloth around it or old hankie &( hold on to cloth so can't end up in drain) push it down drain to see if mold is forming heavy in there. I hate the drain plugs that you can't remove as I've had to take Qtips to dig out hair & mold but that doesn't clean out far down. I'm not at that relative's house enough to know if I might get sick but I do find it disgusting that only way you can clean it out is to disassemble the pipes under sink. I now have a small stainless steel basket in my 1 that is supposed to have that setup as I knew they were impossible to clean. Might put some peroxide or something that is supposed to kill mold down drain for some help but even bleach doesn't do much. Ozone is a joke, I've had 3 cleaners & all they do is make a dirty mess on surface of wall on opposite side of room. Did nothing for my allergies. Friend bought all kinds of expensive "purifiers" & after month or 2 they were sitting there or outside not being used. Might work better for you but I'm allergic to AC,smoke & damp air(high humidity) like before a storm in desert I feel it!! You haven't brought in eucalyptus branches to decorate with or twine plant holders or plants in bathroom (was leaving hospital holding plant & DR. was in elevator & said "Oh, taking home a petri dish," I said What? Dr . said that I needed to take plant outside & plant it not even think of walking through the house with it! He said it was that dangerous! Wow, who knew.I did follow his advise! Got wallpaper on bathroom walls, might be where mold is. I know how hard it is to find out as suffered for yrs & yrs. Sometimes culprit isn't what you think. Good Luck!

Is the bathroom connected to the bedroom? Is there a tub in the bathroom that is never used? If there's a tub, make sure there's water in the j-trap by running water into the tub.

Since you've been dealing with the odor for awhile I would have someone remove the toilet and check the flooring around the flange. It's not unusual for the wax ring to deteriorate and allow leakage of sewage into the flooring.

We had a moldy smell around my sink and it took many months of investigating to figure out that it was mold under the tiles surrounding the sink. Nothing other than a little discoloration--just looked wet. When we pulled the tiles up we found the mold and smell. Good luck!

Thanks for responses about moldy odor and suggestions to help with the problem. Good ideas and I'll try them.

The home inspector sent the home test kit info I had to a microbiologist. Not sure how this will help but I'm willing to try anything. Also bought a dehumidifier and have had it running constantly for the past four days. Humidity is down to 35 % but temp in the room is 90 degrees. Next step could be to open small parts of the drywall in other areas of both rooms.