When your sister has her first infatuation, she is likely to have a strong attraction to the male who catches her attention. Listen to what she's saying, and you'll probably hear exclamations like "He's so gorgeous" or "He's the cutest lad in school." She might even tell some other girl that you're dating! The point is that she is drawn to the boy, but only you can decide if this interest is more than just friendship. If you are not sure whether your sister has a crush on you, ask her directly. If she denies it, there's no need to worry about being thought of as more than friends. But if she confirms it, go with it! Flirt back and let her see how well you both get along.