Children's exemptions are based on custody, not support.
The minor child can be claimed by whichever spouse he/she lived with for more than 1/2 of the year. For example, if you lived together until September, then she and your child lived elsewhere, the child would have lived with her for more than 1/2 of the year. You could still claim the child as a dependent, but only with her consent on Form 8332.
If the child lived with both of you all year and you can't decide, the child would be claimed by the one with the higher income.
Notice that her brother doesn't enter into either scenario.
(Note: If you research the issue, a step child is treated exactly the same as your natural child.)
