Fate / Worm Night - Kotori

From the introduction: In the years leading up to the Fourth Holy Grail War, Tokiomi Tohsaka had a difficult choice to make. For you see, he had two children, two wonderful daughters who were both acceptably talented in the arts of magecraft, Rin and Sakura. Normally this would not be much of a problem, but Tokiomi was a Magus. Only one child could inherit - the other was either useless, or a threat.