It doesn't matter. For tax purposes, a biological parent, legal parent (adoption, married stepparent), and a foster parent have the same tax rights and are counted the same for the dependent test (however, the foster child must be placed in the home by the state or by an agency approved by the state. An unofficial placement doesn't count.) For tax purposes, a child can have more than two people who qualify as "parents" under these rules. That is not unusual.
If you are any kind of "parent" according to the tax rules, then you move on to the other tests. For a child under age 19, the most important test is usually, did the child live with you more than half the year (or, if they were born during the year, did the child live with you more than half the time since birth). For example, if the child lived with the biological parents or other relatives from January to July, and was placed with you in August, you are a "parent" for the dependent test, but the child didn't live with you more than half the year, so you generally can't claim them. Turbotax should walk you through the necessary questions.
This answer has been verified for accuracy by an Intuit expert employee
