It's not master and slave relationship, kids her age were already working and sometimes in much worse conditions.
Shes an orphan and a non-human to boot but the state still funded her upbringing (on an extremely high level I have to add) and got her light labor as a workplace.
She also had the right to refuse. How is this anywhere close to being a slave?
It's much more distressing that she should be more of a guardian to her in terms of age and psychological age difference but she still accepts her as a lover.