Who's the mystery killer?

Luo Shiyi Niang agreed to marry XU Lingyi because she thought the murderer is inside the XU mansion. 

I have two theories

1)If the screenwriter has a good detective style of writing, the murderer is not from XU mansion because in the scene shown it's impossible for any of the concubine to do it because of their location. There is a steep stairs that they have to take and there are several government authorities during that time because they are looking for the pirate who escaped during episode 1. I think the killer might be influential so she would have a hard time getting the justice she's seeking.

2.)if the writer insist to go against reason just to have a killer from XU mansion then the article I've read might be true. The quiet ones are really the most dangerous. 

Your thoughts?

I suspected too of FL stepmother. It's what she said to FL after her mother died that I think it is her. 

My other suspect is her elder sister, but it seems a little more far fetched.

Well if it's the step mom she wouldn't do the deed herself. It would be someone she hired. So she wouldn't need to be physically present. 

There is also the Sea Bann. I am not clear on the politics, but it might have something to do with that too. 

I think the member of pirates meet someone who is influential. And her mother accidently hear something she should not hear that's why she was killed. 

Well the clue is a piece of fabric that isn't to expensive but not too cheap. I think this means it was either a high ranking servant, or concubine. 

Amber is suspicious, but I doubt she is a murderer. More likely she told Big Madam that they had plans to flee. Because she was place with the FL as a spy. 

I sometimes wonder if it was Nanny, but she would have only done it under orders. She doesn't have a person reason to commit the crime. 

It's so early in the show, we don't know a lot about the Xu women. I feel like there are so many more secrets yet to be told.

@leji you mean aunt Qin? Because she's the childless one in the series. I think we read the same article. She has the motives but when did she have the opportunity?

Who benefits from the Sea Ban? We know the ML wants to lift it. There is the concubine who is from the merchant family. She wouldn't want it lifted if her family is making a lot of money off of it.  In addition her son is competing with the Main Wife's son so she would be motivated to keep more Shi woman from joining the family. 

But how would she know that they would be there?

There are also the officials (whose names I don't know yet). They are always fighting about the ban.