Hello. You have a rough right now. I can tell. It can't be easy.
Your mum is depressed and she needs help. This is not going to be easy. You will have to get in contact with someone who can help you - a grandparent, family member, neighbour, family friend. They won't want you to continue living on your life like this. Having to put up with the treating. Hopefully she will get the help she needs and you will become a more happier family.
Why is your step dad a cruel man? You may want to talk to him about why you think he is cruel. He will be in your live for the foreseeable future after all.
Maybe you could have a word or two with your brother. Suggest a few places where he can get a job so he won't need to rob money from you and your family anymore. He could also be used as support for your problem with your mum.
I understand that you don't want to go home. Once you talk to someone about what is going on in your life, hopefully it will get easier and you won't feel this way. I wish you the best of luck.