Can someone please help me deal with stress?

This sounds like a troubling and stressful situation. If your brother is causing so much stress, I feel that you should try to discuss the issue with your mum. It isn't fair on you that he behaves like he does. Your mum is most likely very stressed and tired, so this is why she also comes off as annoying. The best advice I can give is to tell your mum how you feel about your brother, and to remove yourself from the room or situation if your brother and his gf and baby are over. If the baby bangs on the computer box, ask the gf politely to remove the baby from where the computer is, or go to another room and do something else. If you don't have to be in a situation, don't allow yourself to be put in it. Hope that helps :)