I have 2 roosters and 11 hens. Will this be a problem? My roosters are about a year old and my hens are 1-2 years old. The roosters don't fight because they're brothers.

It depends. As they get older they may start getting aggressive with each other, even if they are brothers. Typically you want about a dozen hens per rooster to keep the peace (so they each have their own little harem), and even then there's no guarantee. But they may just be low-key fellows and never fight each other. Keep an eye on them; sometimes the first fight is a fatal one. It's also possible to keep two roosters together in their own bachelor pad, because without girls to fight over they are much more likely to stay friendly.