Curious

I think the son (jae won) must've begged his mother to take tae in as he's his classmate, friend etc etc cause apparently family doesn't approve of this orphan boy and it was conveniently the woman's acquaintance who she murdered . I wanna know why she killed heeseo's mother , does she and Heeseo are related in some way? Maybe Heeseo's mother and her were in a love rival thingy...?? I'm very curious