Skye, good morning, you are not being at all unreasonable to ask her to check with you next time. Who knows what dental appointments, social engagements or whatever a 15 year old has, that she might not remember before making an arrangement. Or indeed that you might have and not told her.
I think you are getting yourself in an unreasonable (i.e. not logical but emotional) tizz about this though. Ok this woman is perhaps not very child centred. And lots of women get a little huffy when their brother marries (a friends daughter wore black at her brothers wedding...) You don't terribly like her. But millions of people have inlaws that they don't terribly like, but put up with because that's what you do. It is odd marrying into another family as their ways are inevitably so different to yours, but your DH of course thinks they are entirely normal.
You can't have it both ways really - in your book she'd damned if she does not pay any attention to your DD and she's damned if she does.
So you have done the right thing, try to look at the positives. There are many worse things that 15 year olds do than go on visits with less than charming aunts. So my suggestion for the day is try to think about the positives in this and not worry away at the negatives. Then you will gradually start to feel better.