Sibling with intrusive thoughts

Hi everyone, I'm here to ask for advice and possible support. For context, I have a sibling, he's 14 and has struggled with mental health for a while, he doesn't get out of his room and barely has any friends. He isn't in good terms with our mom since she nags him to help around the house and she sometimes yells at him but she does it with every one of my brothers. I'm scared for my mom since he has expressed violent thoughts and has vividly described how he wants to k*ll her once they're back in our country and that he's gonna call the hotline. I've never had someone tell me they want to kill anyone and I don't know what to do, I'm just very scared.