I don't know man. This is tricky. On one hand you do pay rent and it's to a family member, so I do think you have some right to expect a housemate to carry their weight when it comes to chores and stuff. On the other hand, it's your brother's relationship and he chooses to be with her, so if he doesn't have a problem with who she is and what she contributes around the house, it's probably not for you to say what she should or shouldn't do. I'm not sure where you should be sitting here.
Have you talked to your brother about this at all? And if so, maybe something could be negotiated? Like maybe a rent reduction or something if you're the one doing most of the work around the house?
As far as her not having a personality and being difficult to talk to. That one is pretty easy. She does have a personality and maybe even a great one. But you said you didn't think much of her from even before you moved in? Well, the feeling is probably mutual and she doesn't think much of you, which could only be compounded if she sees you as an invasion of her and your brother's space/relationship (which she probably does). Thus she doesn't want to show you a personality cause she doesn't like you either.
Anyways, nevermind bringing this to Sherdog. Take it up with your brother and then maybe her. Just try not to interfere with the relationship.