The child's SSN cannot be used on two different tax returns. If your mother files first and claims the child, your tax return claiming her will be rejected. You can file it by mail and let the IRS sort out which of you could claim this child as a dependent. Be prepared to show proof that you provided over half the child's support-----you may need school records, proof of paying for medical care, anything you can gather up to prove that you cared for your sister.
That being said....we have no details from you.....so we do not know whether you can claim your sister.
IRS interview to help determine who can be claimed:
https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent
https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
