The story starts off in Episode 1 where Bosco’s adopted mother was already part of the clan. The clan’s goals were to trick people’s money through deception and manipulative trickery and other bad ways. Bosco’s mother lures Shirley and Bosco (when they were young) away from their parents and captures them with a bunch of other kids. However, she ends up freeing them because she wants to leave the clan. She doesn’t want to do what she is doing anymore. We also later learn that her son has also died probably due to all the bad things she has done before and now feels remorse.
When they escape, they weren’t able to find Bosco and Shirley’s family anymore so she decides to bring them up as her own children..

She swore she would never use any trickery or walk through the path of her old ways and try lead a normal life, hiding her identity. The clan is still after her because of her betrayal to quit the clan so she remaining hiding..

They are VERY SCARY because they’ve pretty much mastered it..we only know around episode 10-13ish that Sammul is their newest tool to deceive rich girls..

“Si-Jie” refers to someone who belongs in the same clan as them, but is higher up in position (usually elder one).
Since Bosco’s adopted mother used to be part of the same deceptive, scheming group as which Sammul is currently in, Sammul decided to show her some “respect”, calling her as his “Si-jie”. In a way it was revealing his true identity to her and achknowledging that he knows who she exactly as is well.