My relative's wife died when the children were small. Then he remarried and that second wife became a beloved person in the children's life. How do I show her as a step-mother?
Go to your relative spouse and edit the person, Then you will see add Parent at the right.
Hope that will be helpfull.
The following article found in the Help Center is beneficial in how to show the relationship of the step- mother to the children after the new marriage is added.