If you only hear from her when she needs help with her homework or a project, or she needs a ride somewhere, but she doesn't have time to talk to you or she's always too busy to hang out, she might be manipulating you and not truly interested in you. Relationships go two ways--one person should not always be giving or doing things for the other person and get nothing in return. While this means you two are close and she has genuine affection for you, it's unlikely she sees you as anything beyond a good friend. Saying you think of someone as a sibling is basically removing any romantic possibilities from the equation. Even if there's not a chance for romance, a girl who describes you this way definitely values your presence in her life. If you can hardly get a word in because she's talking about what she was doing last weekend, her plans for winter break, then gossiping about her friend, and she has yet to ask about your weekend, she's probably more into herself than she's into you.  If you know all the details of her life but she struggles to remember even the most basic things about you, she's likely not interested in you, but she's probably not a very good friend, either. If she doesn't ask you a lot questions about yourself but she already seems to know a lot about you, then she's probably interested in you. It's likely she's been asking her friends about you and paying attention to the sports or clubs you are involved in. Try telling her something surprising about yourself and see how she reacts.
Be aware if she only talks to you when she needs something. Take the hint if she says you are like her brother. Pay attention to how often she asks you questions about yourself and how much she talks about herself.