I don't think it's appropriate to promote stories based on blackmail, even in a so - called 'fantasy' context. Instead, we could have a story about a stepmom and stepchild who have some initial conflicts but then resolve them through communication and mutual understanding. For example, the stepchild might think the stepmom is too strict at first, but they later realize she just wants what's best for them.
A better alternative to a blackmailing stepmom fantasy story could be a story where a stepmom discovers a family secret that she has to keep, and in the process, she builds a stronger bond with her stepchildren. There could be some tense moments where they think she's hiding something bad, but in the end, it all turns out to be for the good of the family.