You and the mom should agree that only one of you provided the majority support. Don't both claim her and have it come back to you.
It also may be a personal thing, where mom may not want her child's exemption anywhere but with her.
Curt Harrington Patent & Tax Law Attorney Certified Tax Specialist by the California Board of Legal Specialization PATENTAX.COM This communication is general information and not legal advice, and does not create an attorney-client relationship. This communication should not be relied upon as any type of legal advice. Please note that no attorney-client relationship exists between the sender and the recipient of this message in the absence of either (1) a signed fee contract and (2) remission of an agreed-upon retainer. Absent such an agreement and retainer, I am not engaged by you as an attorney, nor is any other member of my law firm.Ask a similar question
Since your sister is her actual parent, you can only claim her as a dependent if your sister agrees to not do so. If you both do it, you will lose. But since the child lived with you more than half of the year, and it is your niece, you can claim her as a qualifying child and everything that comes with such as Head of Household. But a non-parent is only allowed to do this if the parent that the child also lives with agrees to not claim the child.
Your sister is entitled to the deduction as the parent. She can release the deduction to you if you qualify as a qualifying relative. You both can not claim her in the same year.
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It would be best if they could all privately agree who gets to claim the exemptions before they all file. Form 8332 could be used, if appropriate.Ask a similar question