You may have a valid claim based on the circumstances, the issue will be proving it to the IRS. You need to prove that your nephew lived with you more than half the year. In addition to whatever personal proof you have, the IRS also likes letters sent to you about the child by reliable third parties, like letters from doctors or the school district about the child sent to you.
Then, if you can prove that the child lived with you more than half the year, you also have to prove that at the time, it was not a temporary absence from the mother's home. This will be harder to do, and will depend on any letters and statements from the mother, the involvement of other family members and whose story they would support if asked by the IRS, and the judgment of the auditor.
