Since she talked about trying to distance her and ML, and talked about the Forest that no one goes to, im guessing she's going to pretend she killed ML with her butterflies, sneak him out, hide him in the forest and then she will escape to the woods with ML. But the knights wont go to the woods cus its creepy and all that. Next, they will go on their seperate ways. And since she escaped, the black haired sadist bro wont meet her for a long time. He'd probably go looking for her to make her cry or whatever. And usually if a character stays away from yhe main character in a romance manga, they'd probably not be the ML.
(this is my opinion) Also, if the brother actually loved her, then why would he want to see her cry. Besides, im assuming they barely met each other, seeing how she was so suprised she met him. Sure, maybe they did see each other, but im pretty sure they never talked to each other after that incident when she was forced to kill the bro. If it wasnt that, then its not love, probably more like an obbsession. And remember , the family is full of crazy ppl. Even including the FL. Ofc there'd be a sadist among them -_-'