Can Taking Herbs Confuse Marijuana Test?

My sister in law was recently let go from her job because a test she took said she had traces of marijuana in her system. She said no that they were herbs. Which herbs would cause this condition?

It depends on which chemicals the drug test considered positive proof.
James Duke's ethnobotanical database (https://www.ars-grin.gov/duke/) lists the following chemicals in marijuana (Cannabis sativa): apigenol-o-glucoside, cannabisativine, cannflavins, edestin, glycocoll, isovitexin-o-glucoside, orientin, oxyproline, as well as others.
You could use Duke's database to try to match up some of these chemicals with chemicals in the herbs that your sister-in-law has been using. And ask the drug testers which chemicals they consider are present in a positive test.