This, among other reasons, is why I suggest to never record a person as a step-parent or child in FTM. You end up with relationships that don't make sense and wrong sets of step ancestors and step descendants appearing in reports - and they are not described as such in those reports.
The fix to this problem is NOT to go to the Relationship button in the Person page, but rather to select either the step-child or parent and go to:
Main Menu > Person > Attach/Detach > detach and detach the step parent from the children, so that the step-parent does not show as a parent and only NATURAL blood parent-child relationships exist.
Setting up step children in FTM will lead to unanticipated consequences, as you have discovered.