Done correctly, if you did list your brother and kids, it would eventually tell you (after a lot of Q&A) that you can't claim them as dependents.
Now factually, you probably can claim your brother's children as dependents, if all of the following are true:
1. they lived with you more than half the year
2. they don't earn their own support (most kids don't)
3. they are under 19 or full time students under 24
4. you earned more money than your brother (their father)
5. your brother agrees not to claim them.
#5 is the biggie of course; if your brother earns enough money to qualify for EIC and file a tax return, he will probably want to claim his own kids as dependents, and as their parent, he gets priority over other relatives, even if you provide significant support.