The relationship between hemp and cannabis is even simpler than that! Just remember it this way:
"Cannabis sativa" or just "cannabis" is a species of flowering plant that grows all over the planet. Depending on the exact strain, and the growing conditions, these plants create a varying amount of THC, one of the cannabinoids (flower resins). THC has a number of medicinal effects in the human body (and in all mammals), including a feeling of being happy or "high". Some people find that effect unpleasant, and some even find it sinful. In any case, humans call any part of the cannabis plant that isn't THC "hemp". The roots, stems, leaves, even the CBD and other non-psychoactive cannabinoids, are all classified as hemp.
So hemp isn't a "cousin" of cannabis, they're the exact same thing. "Hemp" is just a slang term people give for cannabis without THC.
No disrespect intended and I hope you don't mind me dropping that info there. :)