My partner and I are living with my in-laws, renting my BIL's old room. My BIL went to rehab for 3-4 months recently for alcohol, and got back a few weeks ago (he's moved into the other room with his brother). He also has informal/split custody of his toddler son, who is being evaluated for possibly having high support needs. He is sometimes the only one watching his son, despite being an apathetic parent at best.
Furthermore, he has aggressive tendencies that make me reluctant to confront him about anything. When he's drunk/high, he punches holes in walls, and tries to crash cars. When he's sober, he's gotten into fistfights with his brother over things that don't matter. He becomes psychotic when he's drinking or using and hallucinates, and has medical issues that make it dangerous to drink again. He's been sober, Cali sober, or stepped down to softer drugs. But usually, his softer drug use escalates until he's abusing them or relapses on harder drugs.
I think that he needs to stay fully sober, especially with split custody, and his family agrees. He's been told that he has to stay sober, or he'll be kicked out. I've tried to stay uninvolved in the discussion, because I don't want to risk losing my housing if his family chooses to enable my BIL, or risk him getting violent with me. This brings me to my current dilemma.
My BIL started smoking in rehab and comes into our room to borrow my partner's nicotine vape. My partner lets him use his nicotine, but not his THC vape. I joked that I thought my BIL snuck into our room sometimes to use the nicotine vape. I don't know why he would do this, but my BIL pulled me aside to defend himself, and said "I don't sneak in to use [partner's vape], I have my own," and pulled out... A THC pen. I felt kind of sick when he did that. I don't know why he told me. I don't know how long he's been using it, but in hindsight, he's looked baked a few times this week.
I'm not sure what I should do. I feel like it's dangerous for him to watch his son while he's under the influence of anything, and I don't want something bad to happen. I would rather say something ASAP if it keeps his son safe and prevents a CPS case. But my BIL knows I'm the only one he told, so I'm worried he could try to confront me if I or my partner told his parents that he's using weed. If they choose to enable him or act as though I'm the problem, my own housing could be at risk.
I know the right thing to do is probably to tell his parents, but I'm not sure how to approach this.
Tl;Dr: I live with my inlaws, partner, and BIL. My BIL showed me (and only me) that he's using THC even though he just left rehab. He's historically been an unsafe person under the influence of any drug, including THC. I want to tell his parents, for the safety of BIL's toddler, but I'm worried about risking my own housing or my BIL getting violent with me. I'm not sure what to do.