Dear Sister, I've Become a Blessed Maiden Vol. 1

When one man takes his sister’s place as a blessed maiden, his body becomes a magical tool for one arrogant sorcerer in this sexy Boys’ Love fantasy!
It had started as a normal morning. Umehiko and his little sister, Momo, were getting ready for their first day at school and work when a mysterious symbol appeared in the sky. Next thing the siblings know, they’re in a strange world that’s definitely not their own. But Umehiko is only concerned with making sure his sister gets back home safely. Unfortunately, right as he sends her back into the magical seal, he’s pulled back and forced to stay! Theodorus, a powerful sorcerer who’s been working tirelessly to summon a blessed maiden, declares that Umehiko will become his spring for unlimited power and puts a strange symbol on the confused man’s body. Suddenly, Umehiko is overcome with an uncontrollable lust—that only Theodorus can satisfy. Can he endure being the sorcerer’s new ‘blessed maiden,’ or will he manage to find a way to escape?

Yuiko is a manga creator in Japan. They are known best for How Do You Say "I Love You" in Japanese?! I'm Being Pursued by a Hot Korean Actor! and Dear Sister, I've Become a Blessed Maiden.