I´m curious to know. Does this mean you would never, ever, under any circumstances divorce your husband?
While no one could ever take a parent's place, I'd certainly want my kids to have as close to that relationship as possible with someone else if I were not able to be around anymore. I'm not sure exactly what you mean by "there may be a step parent in the future but they will never take the place of us." You wouldn't want that person to step in and fully take the parent role and love the children as a parent would? Or just that you would want to make sure that the other parent were never forgotten. Would you not allow them to call the step parent "mom" or "dad" or refer to them as their parent?
Without going into specific details, we are very religious and divorce is not an option for us. Should one of us die, I would love for the step parent to be involved and loving. However, they would not hold the "title" of mom or dad. They would be stepmom or stepdad. We have had the discussion and we have come to the conclusion of what is right for us. I understand that does not mean it is right for everyone.

It's very sad.