Apologies but Charlene will "not" become the stepmother of Jaz or eric as she has not nor will I supoose adopt them legally.Charlene will be the wife of their father.If she becomes their stepmother which is a common misconception then she also has rights and responsibilities over their upbringing equal to their mother's rights.
I think that we have different definitions of "stepmother" working here. I am using "stepmother" in the aspect of "the wife of the biological father who is not the biological mother of the child in question."
My grandfather remarried well after the death of his first wife, who was my father's mother. Although my own father was, at that time, himself married and raising a family, my grandfather's wife became my father's stepmother.
"Stepmother" in no way implies, suggests, confers or provides legal responsibilities on an individual. It is an appellation, nothing more and nothing less.
"Stepmother" does not mean a "plus mom" or "extra mom."
Charlene, as the wife of their biological father, becomes the stepmother to these two children. Exactly what that means in this situation is entirely up to the parties involved. In particular, it will be the decision of the primary parent, the mother of each of the children, to decide the level of involvement of Ms. Wittestock in the life of her child.
(Right now, the turgid little drama of Jude Law and Sienna Miller is following this line, vis a vis Jude's children with former wife Sadie Frost, and Ms. Miller's liberties with Ms. Frost's children playing out on these lines. One all can't be as adult about these things as are the Crown Princely couple of Norway, sadly.)
If this is in any way unclear to you, please let me know. I've tried to amplify my post so as to avoid confusion.