In the book “addictve thinking” by rabbi twerski, he says that an addict’s friend/relative has the 3-c misconception. One thinks s/he has the power to cause, control, cure. But in truths you did not cause, you will not control, nor will will you cure the addict.
In other words, there is nothing you can do. SB, the best thing you can do is avoid the person and protect yourself. I don’t know this cousin of yours, but I fear he could get physical. My advice is to stay away.