I 36m suspect my brotherinlaw 48m of poisoning his son 18m

About 13 years ago I had a conversation with my brother in law in which he unexpectedly brought up that he poisons his child whenever the child gets on his nerves and that he would never give him too much to kill the child as he isn't a monster. He stated the poison was tasteless and odourless and small doses were undetectable in blood tests. He then backtracked and said he was joking and would never do that. I discussed this with my partner at the time who said he had mentioned trying to design rodent traps. My sisters child has had stomach issues all his life. I mentioned this to my sister and suggested a urine test should confirm if the poison was or was not present. My sister then had an argument with her husband in which he denied poisoning the child. The child has just turned 18 and still has stomach issues and displays symptoms that could be related to mild poisoning. The stomach issues have caused him to pull out of school and remain undiagnosed although multiple tests have been ran. I don't want to ruin my relationship with my sister who has been a huge support to me in life. My brother in law is a manipulator and schemer which has led him to be wealthy, and my sister admits at times she wonders if difficult situations he places her in are deliberate and he doesn't have a great relationship with her or the kids. How do I suggest a simple urine test could remove all suspicion or confirm my fears?