Like I said, it's complicated. Support is only one of the rules for claiming a Qualifying Relative dependent*. An additional rule is that the child cannot be the qualifying child of another taxpayer. If either you or your grandmother have enough income that you are required to file a tax return, your brother cannot claim his niece, because she is QC of another taxpayer.
*A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:
1. Closely Related OR live with the taxpayer ALL year
2. His/her gross taxable income for the year must be less than $4,050 (2016-17)
3. The taxpayer must have provided more than 1/2 his support
In either case:
4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying child of another taxpayer
