@LuminousAzael As she said, she doesn't want to get married. She doesn't want anything to do with it, and more importantly she personally knows someone who genuinely wants to be with him. That, and her final words say it all. Why get married if you have enough money to live off of? Sure, she doesn't have it now but she already knows what she needs to do in order to start making a living, only this time she planned on avoiding men altogether while working. Thing is, though, that like other's have pointed out she unintentionally presented herself as an option anyway with that last line and the things she said about love. Her indifference is in his favor, so of course he'd ask her.
@Mivas It's an effort to provide him with options through her own effort, which would hopefully endear her to him. Also, should she offer her sister from the start and he say no, then she wouldn't have any other option except herself and she doesn't want to get married in the first place, so having the book allows her something to fall back on.