I would contact the social networking site and tell them you were hacked, and by whom; do a little "blackmailing" of your own.

You have to decide what you want and be willing to be up front about it; doing things behind your mother's back when you are still her responsibility is not a lot different from your brother hacking into your life. No one who loves you or is responsible for you is going to like your flirting with activities that can be dangerous for you because you do not have the resources to protect yourself. If you can't take care of yourself getting thrown out of your house, you cannot take care of yourself.

I would figure out how to protect your online work so your brother cannot hack it; if you cannot do that for yourself/your computer then it's probably not a good idea to be on the computer in public places where people out to hurt you, rather than concerned for you, can do so. I understand big brothers can be a real pain, I have three of them, but you are still a minor, whether you like it or not and there is still a lot you have in front of you to learn that you cannot know now and that can hurt you because of your developmental ignorance (age).