There are no ties of inheritance between a stepmother/stepfather and their stepchildren.
An example of this is if a mans wife has children from a previous marriage. Such children will be his stepchildren. There are no ties of inheritance between this man and his stepchildren. The same would apply if a woman’s husband has children from a previous marriage.
Similarly, if a man has multiple wives, his children from one wife will not inherit in the estate of his other wives (their stepmothers).
All step-relatives are non-heirs, and this also includes stepbrothers and stepsisters. As such, a mans child from a previous marriage may not inherit in the estate of his wife’s child from a previous marriage. The two children will be step-siblings and may not inherit from one another.
Note: Step-siblings (such as the one described above) are different from allaati and akhyaafi siblings, who are rightful heirs of each other. Allaati and akhyaafi siblings have been clearly defined and explained in the post The 3 Main Types of Heirs.