Visine is known to mask the results of a drug test. It reduces the urines bubblyness or something like that but that wouldn't be a problem with a home test. They look for that kind of thing in a lab test. Go to Erowid for more info on that. Also I wouldn't know just how much to use. (erowid)
jesus tits! i feel for you, makes my parents seem supportive of my drug habits. yeah, i'd just refuse to take the tests - she really can't do much about it.