Okay, so 6 chapters in and I think she isn't terminally ill. Based on the details provided "Mia" is being groomed to be Duchess while our FL is chosen "to give birth". If we are thinking correctly, then the Duke only wanted a noble birth free from defects (incest) since if you think about it, Mia and ML are somewhat blood related. It's either the Duke or Mia that will do something for her to die... But in the future, since FL was able to give birth successfully. It really doesn't matter for the Duke who will be ML's wife since the child needed was born. ( ̄∇ ̄")
Bruh this Emperor os a failure of a father that now will think he actually did something right once he gets along more with MC buy in reality his daughter ia dead af because of neglect ( ̄ε(# ̄)Σ
In this case, it isn't possession, the MC is really hid daughter but when she remembered her past lives she forgot her current memories as Lotie