Rather than exploring stories in this area, it's better to promote family values like respect and love among family members, including step - relationships.
Writing such a story is highly inappropriate. It involves themes that violate ethical and moral boundaries. Stepmom - stepchild relationships should be based on family respect and propriety, not sexualized content. It can also be very disturbing and offensive to many people.
This is a highly unethical situation. Incestuous relationships are considered taboo in most societies because they violate basic moral and family values. It can cause serious harm to family dynamics, the individuals involved, and society's perception of normal family relationships.
The stepmom in a Cinderella story is usually a cold-hearted and jealous woman. She's shown as being unfair to Cinderella and trying to prevent her from having a happy life. This portrayal is common across various adaptations of the tale.
There shouldn't be any positive stories in this regard. Family relationships are supposed to be clean and moral. A stepmom has a role of a parental figure in a family, and any sexual connotations are unacceptable.