Kate Crackernuts is a YA novel about two girls who are stepsisters. It's a retelling of a fairy tale, so it has malevolent and powerful witches, but it's centered on the two girls: how they met and became friends before they became related, how despite the Wicked Stepmother's efforts to favor one over the other they loyally backed each other up, and how they went off on an adventure and rescued themselves plus a boy.

I seem to recall the author was a scholar of old Scottish dialect and folk tales; the story is told in a heavy dialect that does take some getting used to. And it's out of print, so you might have to check several libraries or used-book shops. But I liked it.