I at first had no faith in the PCR tests, but with time, seeing my SIL tested repeatedly (work related, probably a dozen times), and always testing negative, I've had to conclude that false positives are uncommon ... at least in the medical environment that administered his tests.
I think you shoulda concluded that false positives are uncommon WITH YOUR SISTER-IN-LAW.
Or something about elephants. Or something.
I wonder what year they stopped teaching logic in schools?