Sachiko and her girlfriend's younger sister, who was stranded at home due to a sudden downpour, ended up having sex until morning... MIAA-280

My girlfriend and I, who are currently living together, are considering marriage. She has a younger sister, Sachiko, with whom she has a good relationship. One day, a sudden downpour caused traffic disruptions, forcing Sachiko to seek refuge at my house. My girlfriend, unable to get home due to the traffic, saw Sachiko completely soaked…