On cold, cold day in winter, probably. Maybe the humidity is just a factor of having nine people living in the house.
Need more info...
What we know:
Forced air
Humidity approaching 50% at times, even with a functional dehimidifier
What we need to know:
Fuel source for heating (gas, oil, electric, combo, etc)
Window style (single glazed, double glazed, solar shield, etc)
Storm windows?
Age of home
Foundation style (slab, pier & beam)
Basement? If so, full or partial, and does it seem damp?
"Average" outside humidity
Just on the surface it seems like your parents' home is getting moisture intrustion from somewhere, but more info is needed to diagnose.
Brad