This all could have been avoided if she had just took her sister and left with him when he came for her. Either she never brought up the topic of running away with him and bringing her sister because she really was just using him and she was saying what he wanted to hear. Or author was just lazy with the plot and didn’t even consider this an option. Joining a monastery was stupid because it took her away from her beloved sister and I’m 99% sure they would have sold her little sister to the highest bidder in like a year tops.
instead of saying she’s weak or powerless, I will say she hasn’t been making smart decisions （￣へ￣）
I think the idea is that her sister is in the father's clutches, like he can do whatever he wants with her whenever.
So firstly, I wouldn't say the FL is "weak", it's more like she's powerless. She's a very strong person that has endured a hell-like life for the sake of her sister. She's been abused physically and verbally since she was a child and because she's pretty she's always had men lusting after her and people assuming that she's a whore and uses her looks to get things. Since she's a "noble" (in name only), pretty much everyone around her assumes she's a sheltered princess, which couldn't be further from the truth. The entire story is just *riddled* with people assuming things about her.
People in the comments seem pissed off that she used the ML, but she had no choice. It was that or marry an old man under threat that if she didn't, her sister would have to. Her real plan was to have the ML take her virginity so that her betrothed would get pissed and call off the engagement. She knew her step-sister wanted to marry her off for the connection, she wanted to get revenge on her family by ruining that connection. She wanted to run away from the situation so bad that for a second she panicked and ran to the ML and asked him to take her away. She didn't know his feelings and was surprised when he very quickly kissed her and made his feelings known. He told her he can't take her away, but that he would kill her betrothed. She didn't take him seriously. She assumed he wouldn't be able to, she was just planning to get caught during her wedding night no longer being a virgin and getting revenge when all the sudden her betrothed and his son are dead.
So she used him to take her virginity and he jumped at the thought that she loved him. Which she did and does, but he didn't realize that she didn't *just* use him, it was also exactly how he thought. She tried to explain that she can't leave because of her sister, but he left her there feeling betrayed by her thinking she *just* used him. He still doesn't know how she feels and now someone is trying to force her to make him fall out of love with her so there's more to misunderstand now in how she's acting.