King's Proposal, Vol. 7 (light novel)

Sister Versus Sweetheart! Just as Mushiki and Ruri start to settle into their normal routine again, their uncannily talented older sister, Madoka, decides to pay a sudden visit to the Garden! When pressed for the reason he transferred schools, Mushiki reveals he’s fallen for someone. In order to convince his sister to accept his choice, however, he must introduce her to Saika Kuozaki. The next thing he knows, Mushiki is showing Madoka around Void Garden’s annual school festival, where he manages to greet her in Saika’s body…yet somehow, Madoka instantly sees through his disguise! To convince Madoka that Mushiki is telling the truth, Kuroe has no choice but to introduce herself as the real Saika. But Madoka’s standards are as stringent as they come, and there’s only way she’ll accept her darling brother’s future wife: trial by combat! It’s the world’s strongest mage against the world’s most protective guardian, with marriage on the line!