A step child is considered the same as a biological child for tax purposes. That is, if the child lives with you when she is not a school, you may claim her under either the "Qualifying child" rules or the "Qualifying relative" rules.
See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
You may only claim the education credit if she qualifies as your dependent. If she is not your dependent, the parent that does claim her as a dependent can count the money you sent in claiming the credit for themselves.
