Ok, I am of two minds
Scenario 1 - I was thinking that it may be the FL's step mom or her daughter - because having her run away would ruin their plans.
Scenario 2- Somebody from Xu household was having a cladestine meeting with her lover/accomplice/ whatever and was afraid of being seen. My suspects are Concubine Wen or the elder sister in law. The one who's husband passed away. She seems way too smart, but quiet.
I do think we need to add the FL to the list of victims in scenario 2 because it was just so suspicious how she got sick for a long time and just never got better. I suspect someone in the household was slowly poisoning her.