If you are married to the other parent, you are the child's step-parent. A step-parent has the same legal status as a biological parent as it comes to dependents. In Turbotax, if you are preparing a joint return and it asks for the child's relationship, choose either "child" or "step child", don't choose "someone else."
Separately, when a child turns 17, they no longer qualify for the $2000 child tax credit, only the $500 "other dependent" credit. This is part of the 2018 tax reform law, although it has older roots as well.
