Nah her husband loves her too much to poison her
It might be that she was sick because she was poisoned and she was poisoned because she is the only remaining viserade woman in their family. Only the woman in viserade can become an empress and it might be the reason why those who are against the emperial family are so persistent on killing her. As long as she is alive, the viserade family will always be at the 2nd highest pedestal next to the Empire since anytime, she can be summoned as an empress or an acting 2nd immediate guardian of the crown prince, which is his nephew.
Wait is her husband behind her illness?