I currently have custody of my niece. She lived with me from the end of December 2009 until the beginning of May 2010. In October of 2010 she was given to me from CPS (now officially a foster child). I claimed her on my taxes and it was rejected because someone had claimed her already. My brother (who had 100% custody, the mother has 0%) knows I was claiming her. Is there anything I can do? I don't think I can fight it because I can't prove she was with me in the beginning of the year.

Call CPS and ask them to provide you a letter confirming when you were given financial responsibility for the child and take it to your tax preparer. You can also call the IRS helplines and ask how to handle it- they are actually very, very informative and I've had a great experience using this service.

If you provided over half the child's suport, then you can claim her as your dependent even though she didn't live with you all year (a niece can be a qualifying relative without living in the same household). You can also claim her because she is now your foster child, and hence a qualifying child. The only person who could fight you on this (and possibly win) is the custodial parent, which from Jan - Oct was your brother. But you say you and he are in synch on this, so that shouldn't be an issue.

My suggestion is file your return claiming her as your dependent, but you will have to file by paper (not electronically). I suggest you include a letter indicating that you are the child's foster parent and that you provided over half her financial support in 2010.