Hard one for anyone outside the family to judge, I think.
It sounds like you had a firm arrangement for the weekend in question, arranged months in advance. In which case, yes, she is being unreasonable.
Do you let her know clearly, and in advance, what hours she is going to be expected to be available to you? If so, then she should also have the courtesy to agree in advance when she's going to be committed to you. If you have a more flexible arrangement, with you calling on her 'as and when', then I think you also have to cut her some slack too. If, for example, you're expecting babysitting at short- or no-notice, then I think you cannot expect much in the way of accommodation from her, either.
Pointing out mistakes and her getting cross... depends what you're pointing out and how. If you're doing it nicely, and it's stuff any sensible person should know, then she is obviously being unreasonable. If it's stuff that you're being picky about, then maybe not so unreasonable - see, it's impossible to say without knowing more context.