If you and your spouse are filing a joint return then the children can be claimed as dependents on your joint return since you say they live with the two of you for more than 50% of the time. A step-child is treated the same as your own child on your tax return. If you filed a joint return for 2017, then the AGI is the same for both of you when you enter it for your 2018 return in order to e-file 2018.
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