It is absurd to believe you may correct a biological illness such as diabetes, schizophrenia, or bipolar disorder with talk, although addressing social complications may be helpful. Symptomology may change overtime while the underlying disorder remains. You should request the diagnosis and its explanation from professionals. Mental health professionals have varied degrees of competence. You have a right to ensure your personal safety.
Strange behavior is a symptom of the disorder.
Don't take it personally. Don't be afraid to ask your sibling or parent if he or she is thinking about hurting him-or herself. Suicide is real. Don't shoulder the whole responsibility for your mentally disordered relative yourself. You are not a paid professional case worker.