Head on down to the Older Children/Teenagers thread for some hints and tips if you like.

Do you have any other support? Local council can be a good place to start. Speak to your brothers school and let them know the situation, they may be able to refer you to some sort of program for teens. Your brothers may have some issues with mum letting them down, and it will be hard enough for you to deal with that as well as getting them to do homework and go to school.
A family counsellor would be valuable to help you set ground rules, boundaries and expectations for everyone.

I'm not sure if you will find a forum with this situation as it is a little rare (good on you), but if you are taking on a guardian role, the issues would be much the same.

xoxoxxo Good luck!