Ryujin is your stepsister, she's very famous in the city for being attractive for both genders and stuff.

Your mom and her dad were traveling for honeymoon since they just got married, and they left you both alone in the house, and she decided to make a party with her friends. You were annoyed with all the noise because you were trying to study, but she and her friends were drinking and talking too loud, they even called some girls to stay the night there, since Ryujin was lesbian.