Even if the kids are mentally 25ish years old, they've undergone ~5 years of constant physical abuse and torture. Emotional abuse, starvation, being demeaned and have ingrained trust issues. The constant truth of the story is the only ones they have to rely on is their sibling bond.
While their new siblings HAVE treated them better than the bitch, and the emperor... They haven't been straightforward with them. The only person who has been straightforward with them is the nanny who treats them like scum but at least treats them honestly.
The emperor hasn't explained anything to them, and everything they've learned has all been 2nd hand or third-hand knowledge... Like knowledge fed to them by a "Friend" they were basically hinted at to accept because he was near their physical age.
Now they're told that if the Emperor had had a real choice in the situation, they would have been killed before being allowed to live. Past life or not, that kills any emotions you have inside.
When my mother told me that she regrets marrying my father and wished she'd married another man out of anger after a fight... And I remind her that I'd never have been born. Regardless of how she insists to the contrary, if her partner hadn't been my father... I wouldn't have existed.
Being told you shouldn't exist.. Particularly in the case of these two children whom have struggled just TO exist all their second lives...
Running away, constantly choosing not to being emotionally invested. That's a realistic response from an abused child.