Will the prosecutor go lightly on us if the drugs we were selling are for cancer

For the prosecution to drop the kind of charges that you are indicating in this question, there would have to be really good reason. We do not think that for your relative's claim this was for cancer will help at all. The prosecutor will undoubtedly tell them that they could have gone the route of every other cancer patient and gotten the drugs legitimately. Our opinion is that they will go hard after them in prosecution and likely both will do time (especially if their are any priors). We do not think they will mercy nor will they think this excuse is worthy of leniency. But, this is a guess, we are not privy to all of the information in this case.