Sister-in-law, sister-in-law

Ryoko and Ayumi are close-knit sisters. They all got married and left the house, but they divorced and returned to their parents' house. It's been a while since the sisters were drinking alcohol and talking about memories, when their younger sister Ayumi invited them to take a bath together like they used to. Perhaps due to alcohol, the younger sister asks her to show her how she masturbates. My sister was confused, but gradually she became serious about fingering, and the younger sister who was watching it became so excited...