“This is the mode we will be in for some time,” said Sam Kamin, a law professor at the University of Denver who studies cannabis law. “As marijuana becomes more legal in more states for more purposes, the tension with the federal law will become more pronounced.”

The hazy legality of hemp can be seen in products like hemp granola and shampoo, which are allowed to fill health food store shelves even though they technically violate federal drug laws. All those products are made from imported hemp, which has generally been permitted into the country so long as it has less than 0.3 percent THC.

Image Mr. Mattison prepared the treatment for his daughter. Credit... Matthew Staver for The New York Times

If the Stanleys ship their oil, industry watchers say, it will be the first time in decades anyone has tried to sell domestic hemp nationwide.

In recent years, hundreds of families with epileptic children have moved to Colorado to try oil made from the Stanleys’ shrubby strain, which they call Charlotte’s Web. The national Epilepsy Foundation has called for it to be available to all patients, though formal research into its effectiveness remains scant. There is a nationwide waiting list of more than 9,000, which the brothers hope to eliminate by expanding their crop from small greenhouses into vast hemp fields.

“We are hoping the enforcement agencies have bigger fish to fry and don’t want to take a bunch of medicine away from sick kids,” Mr. Stanley said. “But if they are going to do it, we’re all in. If you are going to be locked up, it’s a thing worth getting locked up for.”

The brothers, who had a Christian upbringing in conservative Colorado Springs, started a small medical marijuana business in 2008 after seeing the relief it brought to a relative sick with cancer. At first, they grew mostly marijuana high in THC that packed a serious psychoactive punch. On the side, they experimented with breeding plants low in THC but high in another cannabinoid known as cannabidiol, or CBD, which scientific studies suggested was a powerful anti-inflammatory that a handful of small studies showed might have potential as a treatment for certain neurological conditions, including seizures and Huntington’s disease.