Wait...you just adopted her...why would you be sending her back to a place you know she is being mistreated? Hell, the Marquis could end up killing her just so you can't get her.
I'm not sure if her threats to the older sister hold water either. I wonder if the older sister is too old to sacrifice now. Plus the parents seem to have actually loved her for her entire life. As evil as they are, maybe they could do it, but they would just have to forget all that love they had for her and switch it over to the daughter they treated like less than a dog for 12 years. It would be harder to play off the older daughter's death as an accident too, especially since she seems to be engaged to the prince, and unlike the sacrificed children in the past, she has been a social butterfly whom everyone would know well. A lot harder to sweep that death under the carpet.