Sister! No One Hurts You with Me Around!

Clara Sinclair brought up her younger sister from the time they were both girls, and then learned the girl is not a Sinclair at all. The daughter born to the family, Iris, was swapped at birth and grew up outside it, badly treated for years. Clara goes and gets her. Iris arrives frightened and behind on her schooling, so Clara puts her into classes and stands over the household while Iris learns what she was born to. The girl who has been living that life does not step aside for her. She sets things up to catch Iris out, and then to catch Clara out, over and over. Clara works backwards from the swap itself. The trail goes to her uncle, who arranged it and has been directing the sister ever since. He keeps at Iris, and Clara takes the schemes apart one at a time as they come.