Oswald’s madness

"It's because even I don't know much about Golden Poison. He formed that gang to make it as a source of income for the research he was trying to do on the island. I didn't know about that until recently," William informed Aiden.

"What happened after my mother went to meet Oswald? What deal did she make with him?" Aiden asked.

"Your mother agreed that she would work for him along with your father but they won't be a part of the zero team members anymore. They would decide their own fate and carry on with their lives like a normal family. Oswald didn't want to do that deal but your mother was stubborn.

I am not sure how she made him agree, but in the end, he gave in. I think as Oswald had groomed her personally, he was slightly attached to her. He assigned them to teach the new zero team members their tricks and also the skills.