Wait, so let me get this straight: the parents knew that Onee-san had the hots for Minoru and that she was filthy rich, so they were counting on her taking care of him when they abandoned the boy in hopes that Onee-san and Minoru got married and the parents could them get access to her money through Minoru? If so, did they discover that she was rich when they moved in the apartment and tried to take advantage of that, or were they actively looking for single rich ladies that they could marry Minoru off to?
Anyway, I'm glad Minoru decided to stay, I'm scared to know what they would have done to him if he had gone to Brazil.