I don't know how else to say this but if you subbed with someone's urine to hide your pot smoking and it was positive for morphine, the chances of it coming from valarian root or St. John's Wort has to be a million to one if not worse. Despite what you may have read on the net, if the sample was confirmed using GC/MS there is no way GC/MS would confuse the metabolites of morphine (morphine glucuronide) with the metabolites of the active chemicals in valarian root or St John's wort. Your body can't change the chemicals from the herbal supplements into morphine glucuronide no matter how hard it tries. These websites that talk of false positives are usually also trying to sell detox drinks and such for drug tests and by scaring you they can increase sales. Technically, poppy seeds are not a false positive either, they are a positive because they can contain small amounts of opiate alkaloids. You have to consume a rather large amount to test positive from poppy seeds.

Was this a test for high school that used an instant test or for a college that sent it to a lab?

I suspect your brother isn't telling you the whole story.