Your heroine must produce an heir in order to take her rightful place in her kingdom.
Unfortunately, her husband isn’t capable of passing down his family’s magic but his brother is…
What does she do? Does she seduce her brother in-law? Does she use her magic to trick him? What are her plans and does she follow through on them? More importantly, what are the repercussions?
Does she love her husband and despise her brother in law or vice versa?
Perhaps it’s her intent to gain control of the kingdom by using the brother.
Why does she want control of the kingdom?
Will she seduce her brother in law, manipulate him or threaten him?
Does he did or become a prisoner? Maybe he becomes her puppet or confidante.
Maybe these are good ideas, maybe not. But it’s fun creating the possibilities.