In cases like this, it’s likely the victim defended the family, and it made it impossible to classify the crime as slavery if she said she was free to leave but “was afraid of the violence outside”, which the article mentioned. It sounds ridiculous but in any court, if you can’t prove something beyond doubt, you cannot punish, which I think is why they ended up with that arrangement.

If you ever been to or lived in Fortaleza, "being afraid of the violence outside" is not ridiculous at all.