Boku no Yasashii Oniisan

When Satoshi Kashiwabara went to Nichoume, he fell in love for the first time with Nichoume's prince; a known playboy. But right after they kissed, he received a message saying his father was in critical condition and left the hotel. However, a few days later, he found himself living with his step-brother, who turned out to be the prince himself!!! It's a romantic comedy about step-brothers suddenly living together knowing they can't be together.

(Source: Manga Updates)