The quick answer is no. In order to claim your wife's son as a dependent, he must meet a qualifying child test. Although the stepson would appear to qualify as a dependent in 2013, if you meet the other tests (support, age, principal abode, etc.), he does not meet the qualifying child test in 2012 because you were not married in 2012. He is technically not your 'stepson". As such, you could not claim a child tax credit or the EIC either in 2012. But, this could be different for 2013.
No for 2012, not your child or stepchild in 2012. 2013 should be good since married to mom and stepchild.
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I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.