Yeah this is a lot of confusion. So like she's still their daughter but is having a relationship with their son? Oh but it's adoption, so so that makes it.... okay? Like what?? And also what's with the dude who wants to marry her? I'm actually cheering for the villain to marry her because, I'm not comfortable with the brother and sister.
They mention in the earlier chapters how girls get “adopted” into houses in a “daughter-in-law” way, not as a daughter, which is more like setting up a marriage prospect. So, they are allowed to marry under those terms. However, it’s a bit confusing because the father fully intended to adopt her as his daughter, not as a marriage prospect. But they're also afraid of the ML, which is why they don’t object. Still, there were so many gaps, it’s confusing.
.. I see you trying to eat your cake and have it too...
You would like to bang their son sometime in the future but also want to call them Mum and Dad in an unironic way