If you want the extra money, it is a good idea to accept more hours. However, I do understand why you do not want to do her any favors, although, like I said, you are the one coming out ahead with more money in your pocket. So, perhaps you may want the extra income. If that be the case, don't look it as doing her a favor, look at it as an opportunity to make more money.
However, if you do not want to work extra shifts, then you have zero obligation to do so. The only reply you need to make when she requests you cover her is "I have other plans, but thanks for asking anyway". I would be very tempted to say "You know Jane, if you treated me like an equal, I probably would cover you, but because you are rather witchy to me, I feel like doing you a favor is a slap in my face!" Ahhh, wouldn't it be nice if you could say that and not suffer additional witchy consequences???? LOL