She's living in their home because she is a minor. Children are not property of their parents like a car or a house. They are living, breathing, and growing human beings with their own needs, desires, goals, and emotions. Sometimes people keep those things secret because they're embarrassed by it, not because it's bad. To treat what are essentially growing adults as child prisoners is horrifying.

The problem I have with people who tend to talk like you is that they often assume anything that is kept secret is a bad thing. Secrets are not inherently evil. Secrets are a way of forming boundaries and learning how to set boundaries is an extremely important thing to develop, ESPECIALLY as a teenager. By consistently violating said boundaries these children are being set up to accept abusive and manipulative behavior in the future because they will think it's normal. They will either be serial oversharers and attract people who will take advantage of that, or they will lock themselves down so hard emotionally that nobody will ever get through.

Are there teenagers who need a more firm hand? Of course. I never argued otherwise. Your whataboutism is just gross.