My take from the latest preview:
It seems Na Mi was paid to have an affair with Jung Hee. In episode 2, she could be seen reporting to some lady over the phone and had received payment for it. She was probably in it for money for her mum’s operation but somehow had fallen for him. And now she’s having second thoughts about ruining his marriage and wants to confess.
This lady who hired her works for and manages the household of Eun Hee. Could the latter be orchestrating the whole thing to get Jae Bok to divorce Jung Hee and then move into her house? Seems like a lot of effort but for what purpose really?