Why do I feel like Onee-san was stalking Minoru. His parent's caught her in the act. The way they were in the debt, they demanded 30Mil yen for their kid as a joke. Oneesan actually brought already pre-signed papers in a contract. They agreed and disappeared for an unspecified long vacation.
It's still weird, not knowing about the person's name and occupation living so long together. It feels to me that the boy is only complaining for appearance sake and never push too far into it.