Mami and I have been dating for a while, but I rarely get to kiss her, let alone have sex. Today, Mami's parents went on a trip, so she invited me over to her house and I could even stay overnight. Awesome! It's the first time I've stayed overnight at my girlfriend's house, just the thought of us sleeping in the same bed and being able to do 'abc xyz' comfortably already turned me on a lot. But then Mami mentioned she has an older sister named Yume, which surprised me as I always thought she was an only child. When I arrived at Mami's house, Yume immediately started chatting with me. Yume is a very cheerful and friendly person, who likes to ask 'sensitive' questions. I didn't pay much attention until she exposed her breasts and buttocks in front of me, even when Mami was right next to us! I felt quite confused but couldn't take my eyes off her perfect body. While I was talking to Mami in the room, Yume found an excuse to make my girlfriend leave, and she continuously used her body to seduce me. Unable to resist her beauty, I put on a condom and officially had sex with my girlfriend's sister right in her room! But as we were having sex, Yume suddenly stopped and took off the condom. I knew this was very dangerous, but the pleasure of having unprotected sex made me lose all sense of reason. I fucked her like crazy and came inside her pussy over and over again! And throughout the two days and one night at my girlfriend's house, I came countless times inside her sister's uterus while my girlfriend was right next door...