You Call That Service?, Vol. 2 (light novel) — Kisetsu Morita

Nothing like a little sibling rivalry!
Women want him! Men want to beat him! And Ryouta just wants two minutes to himself...But there's little chance of that happening while he's cursed to be irresistible to the opposite sex-especially when two of the girls fighting over him are Ouka, the ruler of the Sacred Blood Empire, and her little sister, Shiren! What ever happened to the Sacred Blooded being immune to the curse?! ...And why is Shiren such a terrible student? While she's busy trying to improve her grades, Ouka has no trouble swooping in and snatching Ryouta away! Is Shiren in danger of losing her minion to his childhood crush?!