No kid should have to live this way. It’s inappropriate and probably illegal. Unfortunately, I don’t know what resources are available to you in Egypt. In the United States, child protective services could get involved as a way to help your brother.
I’m glad you tried to talk to your mother. That’s the first step, but I’m not surprised that it didn’t help. She has quite literally kept you in the dark for years.
You didn’t mention whether you have talked to your father about the situation. Does he know how his children are being treated? Is it possible that he could take your brother to be with him? I suspect your mother will be angry if you talk to your dad. But if talking to him is the only possible way to protect your brother, it might be worth putting up with her shouting.
I’m afraid that I just don’t have enough information about what is possible where you live. Perhaps a relative or your school counselor does know. Don’t feel you need to keep your mother’s secrets. There are some secrets that shouldn’t be kept and mistreating children is one of them.
If there is no immediate way to get help — you can take the long view. Go to school. Stay in regular contact with your brother through social media so he knows he has a sister who loves him and cares. Get good grades so you can get a good job. Then consider taking your little brother to live with you.
I wish you well.