These are spoilers from the novel (or how I understand it) :
The first (original) life:
So the fl thinks that ml wants a strong and independent woman to help him with the empire so she tries to not bother him and basically doesn't talk to him at all. Because of that ml thinks she doesn't have any interest in him but he still wants to marry her because he thinks they could become friends and great partners in the future. When he meets Sylvia (the little sister) he fall for her right away because she seems fragile and pitiful. But he knows he can't marry her because she would only became his weakness (and also he hates himself for the whole fiancée's sister thing). When he hears about Sylvia's death he goes insane. He needs someone to hate so he blames it on the fl. He frames her and causes her death (he's happy when he hears about her death). After her death the ml gets killed by fl's knight who escaped from prison.
Fl's mother:
So all that love for Sylvia is kinda a lie. Because Sylvia is not hers she hates her and feeds her poisonous herbs (she made an excuse that it's a pain-relieving medicine). That's why Sylvia is weak and can't leave the mansion.
Not really a spoiler, but...
In novel, it said the original mc didn't respond to stimuli around her and was only babbling nonsense. Not that she was mute...
That changes a lot. Do you think theyre taking another route or a mistranslation?