[ She is quiet for a moment. Admitting that her sisters were criminals would be acknowledging that they did something wrong; potentially of their own accord.
Another thing to consider is that everyone in this dome would be considered "criminal" by the same or similar standards, even if she has met some truly innocent-seeming individuals.
Subaru's downward glance is a silent admission of guilt. ] I have several sisters who spent some time in one of those centers. I think they benefited a lot from the experience.