So, I get the support towards the bio dad and all. But. She's happily living with duke daddy as is and is much safer there than in the royal palace. Royal dad needs to sort out all his family issues, maybe take control of his god damn place as emperor, as stated below by another lovely commenter.
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Then maybe try to build a relationship with mc, he can't just try force her to go back with him when she's clearly used to and now loved by the duke fam. Also totally get that he'd probably been brainwashed, but come on, mc expects very little of bio dad and brother, plus she's clearly
traumatised .
Would support a start to kindle a relationship later on when she's less traumatised and more comfortable with relying on duke family, cause pushing her to choose is bad. Plus when she knows that royal dad and brother don't mean any harm and actually care for her.
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Honestly trying a co-parenting set up would be a good start too.