Brother keeps threatening to kill himself

My brother is about 15, I'm almost 21. We live in a joint family, and they're inredibly abusive, my parents and grandparents both have destroyed our mental health in more ways than we can imagine. My mother was always extremely incessant on academic performance, and so i somehow managed to get good grades each year, but my brother struggles with it a lot. I think he has adhd, im not sure though. Anyway, my father is a very difficult man, he is emotionally and physically abusive, to my mother, to us, especially my brother. The situation in our house is so awful, that we always live in fear that he might get angry one day and do something to us or himself. My brother has started becoming just like him. Some days ago, the cleaning lady went to his room to mop the floor, it was around 11 am so it wan't super early either. My brother screamed so loudly at her, multiple times, it was absolutely terrifying. This isnt the first time he has done something like this. Now, everytime my mom or anybody else tells him to study or do anything he doesnt want to, he threatens to kill himself. I wish i could do something, but ive no value in this house or in his life cause my father made sure that he never learns how to respect women. I dont know what to do. I miss my little brother. I remember when i used to carry him on my back and play games with him, i miss his laughter so much. But he's so far gone, im not sure if he will grow up to be a good person. I'm so ashamed for not being able to protect him from my parents, but i was a child myself. And now, every night i live in fear that he might not be there tomorrow.