Brother in law stealing prescription drugs

My brother in law has a history with addiction. I am at present living with my sister and her family in their guest cottage as health issues have left me unable to work. I am forever grateful for their help and I don’t know where I would be without them. They are also all lovely people and I am happy to have them in my life. My sister and her husband are much younger than I am and still working. I have lupus and I take several analgesics for pain. Some are opiods. Over the last couple of months I have started to notice that my prescriptions were running out a little faster than I thought they would. It is not really a problem for me since I have a wide discretion over how much pain medication I want to take and I don’t usually use nearly as much as I am prescribed. I just prefer to feel clear headed I suppose even if at the cost of a little discomfort. My brother in law comes into my cottage daily to help me tidy the place up and to help with little odds and ends and we usually sit and have a coffee together after. He usually goes to the bathroom where I keep the pills while he is here. Because I was having some suspicions about pills going missing I decided to count and write down how many pills of each kind I had before his visit and again after. He must have been taking some when he went to the bathroom everyday because when I recounted after he left I saw I was short 4 pills. Now I don’t know what to do. I am so much in their debt that I do not want to make any trouble for them. I do not know if I should simply move the pills to a more secure spot and not say anything or if I should get involved in their lives and talk to my sister about what is happening. I am not an expert on addiction but he did require some treatment in the past about 10 years ago so maybe he does again. To me however, he seems to be acting quite rationally – for all I know he isn’t even using them himself. I am loathe to act the busybody, but as a health issue, does my sister need to know about this, even though it may cause discord in their marriage.