Kids, weed and inlaws...?

So, my father-in-law informed me last week that he has decided to turn the spare room at his house into a pot-harvesting room. I have a strong dislike for drugs, but am not judgemental about it. I am worried that he's working out deals with his kids, where he'll pay off their debts if they agree to buy their weed from him. He began bragging about how much money he'll make selling his 'goods', etc. I just pretended I didn't really hear him. I do not object to people doing this, but it concerns me that he's my future father-in-law. This behavior makes me not want to take future grandkids to see him. He comes by to visit and knows quite well that I disapprove of weed, and does not do it around my family. I don't much want to visit him anymore, let alone what will happen if kids are involved. Am I being overly sensitive? Am I being overly judgemental, unreasonable or critical? I love my fiance, and I don't want to come between him and his dad. Any suggestions?