It's not Lusi. More scenes were shown in ep 7 or 8 that Lusi's mom was pregnant with her when the bandits raided their campsite, stole the valuable tributes, and killed ML's father.
I also like this setup more because if FL's were the daughter of ML's family maid/servant, in historical time, it would be frowned upon for him to marry her because of the big gap in class. Not that he cares if he really loves her, but I don't think the author would set it up like this so be more inline with cultural heritage and practice of the olden days.
It seems like people may bear the family name Cui or Yan, but aren't necessarily blood related respectively. It's more like a "clan" or "house." I remember Cui Shi Jiu addressed a servant whose last name was also Cui.
From Ep 7, ML's had a flashback to the scene of his father's death. We can see that both ML's and FL's parents are dressed very well. I think they are more of a business partners/friends relationship instead of FL's parents working for ML's parents.
Yes it seemed like ML's father sent his wife and ML away whil FL' father sent his pregnant wife away when attacked.…
I don't remember all the scenes, but this should mean ML knew DW was the unborn baby of her mother. DW did say that the red pearl belonged to her mother.