I think the biggest one, and I think it's true for all the de Luce girls, is that they spent so much time at odds with one another. The times in Flavia's canon when they're all getting along (or one of them, at least, isn't being terrible to Flavia) are rare and lovely, and heartbreaking because you know it isn't going to last.

I also think Flavia's going to realize how much of a know-it-all she can be, and how insufferable it is to be on the other side of it.