Most likely not.
The first issue is that you can only claim your daughter as a dependent if her income was below 4,050 dollars and you provided more than half of her support for the year. Since she did not move in with you until September, I assume it is unlikely you would have provided more than 50% of her support for the year.
The second issue is that children are always the dependents of their parents if the following applies:
The children are under the age of 18, or between 19-24 if a full-time college student.
The children live with you for more than half of the year.
The children do not provide more than half of their own support.
For older children, they do not file a joint tax return with a spouse.
The mother most likely qualifies to claim the children. However, if she did not qualify to claim the children or if she qualified as your dependent, her ex-husband may also be able to claim the children if they lived together until July 1, 2016.
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