Compelled.

Later, Robin would think he had suspected before, but it was here and now that the final missing piece of the puzzle slid into place, a new understanding far darker than he could have anticipated lowering onto him like a shroud. It seemed impossible, utterly ridiculous, and yet the wizard knew it to be true with an absolute certainty he had felt only twice before in his life.

"An Imperius Curse," he said at last, more to himself than to his sister. Their father—a man who was now just clinging to life—had cast an Unforgivable upon Miranda, and surely Robin as well. How could this be?!