It would because Elsa is a good person and honestly doesn’t want the title or mansion; it wouldn’t be because they have no blood ties to that house. Unless I’m mistaken Karen is the step child, no?
Karen has the same powers as Elsa's father and Elsa has called her her half sister, so maybe they are blood related
Although Karen does indeed have the flower power of the Sorenburg duchy (as she is Elsa's half-sister, not step-sister), the children wouldn't have the family power since they aren't Karen's biological children. Franz just told them that Karen is their new mother since their actual mom (the shadowy figure with brown hair that was briefly shown) is no longer in the picture.
Tbh I just assumed Karen and silver hair guy had them in secret and neglected them because they were born out of legitimate wedlock, they are those kind of scum after all, I completely did not notice the cloaked figure of their real mom.
Not only that, but Karen was also shocked when she learned that he had kids
It would make sense if the 2 girls grew up to inherit Elsa's family title/mansion and stuff