**Spoiler**
The mysterious girl is the same girl that Sit Ding San loves (Chan Kam Ding)........she is the long lost sister of Yang Fan. He will use her against the Sit family. She will pretend to love Sit Ding San to cause confusion for the family and misunderstanding btw Lady Fan and Ding San.
As for Lady Fan's so-called son will develop to like Sit Ding San's sister.