"And they all should realize how much they are taxing my patience." Casting the diagnostic charm, they were able to see where the curse was centralized in the wound before it began to spread. Twisting the curse so it can recognize medicinal spellwork and retaliate is just... too intelligently evil. She didn't want to analyze it too deeply, but the spellwork was too sentient and too rooted in Necromancy for her to miss her sister's signature.