Those two statements have nothing to do with each other.
1. Injured spouse. The injured spouse form is used if you file a joint return with your spouse, and one spouse does not want "their share" of their refund taken for the other spouse's past debts. You can file an injured spouse form any time you meet the qualifications, the IRS will ultimately decide how to split the money. You never get away completely scot-free, the IRS always takes something. How much depends on many factors.
2. Dependent. A step-parent has the same right to claim a child as a dependent as does a biological parents, as long as they meet the other requirements (age, residency, and so on). If your biological child (who is also your spouse's stepchild) lived in the same home with both of you for more than half the nights of the year, then either
a. you both can claim the child when filing married filing jointly, or
b. either one of you (but not both) can claim the child if married filing separately.
If you haven't filed yet, and one of you has a debt, then you could either file separately, and the spouse without a debt claims the child dependent; or, you file jointly, both claim the child on one joint tax return, include the injured spouse form, and see what the IRS takes.
