First, a step-parent has the same legal rights to claim a child as a dependent as a biological parent, and that rights extends even after divorce or death of one of the parents. So you and the stepmother are in the same legal position with regards to the dependent rules.
With that being said, the usual rules for a child under age 19 are that they can be claimed by the parent where they live more than half the year, as long as they don't provide more than half their own financial support. So if the child still lives with her stepfamily, then nothing has changed even though your ex-spouse has passed. Alternatively, if the child changes her living arrangements, you would be able to claim the child if she lived with you more than half the year.
Lastly, you may be aware of the special rules for children of divorced parents that allow a child to be claimed by a parent they don't live with if the parent they do live with signs a waiver. Those rules don't apply after the death of one of the parents, so you can't use a waiver to claim the child if she doesn't live with you, even if the stepmother was willing to sign a waiver. The dependent claim is now based entirely on the normal rules about age, residency, support and schooling.
