EBWH-233-RM “She’s my girlfriend’s older sister, so it’s not cheating.” During a hot spring trip with my girlfriend and her, she seduced me

My girlfriend, her sister and I went on a trip to a hot spring. It was supposed to be just a fun, healthy, and peaceful trip, but... I couldn't resist my sexual desire when I saw her sister's amazing body, and before I knew it, I was having sex with her while she was sleeping.