[guardian]

Assuming you are correct and the coolant is disappearing without evidence of leaking, it has to be going somewhere. There are two likely places:

Either out the tailpipe or into the oil.

You need to check, or have a mechanic check, and try to detect coolant, or coolant residue, in a place it should not be.

As for your wife's uncle: He did you no favors. And the word "you", in this instance, is plural.