My drunk sister-in-law came to my apartment where I live alone with her friend who missed the last train and we decided to sleep in a riverbed position! As the three of us slept in a riverbed position, my sister's friend's clothes came open, giving me a glimpse of her breasts and panties! I couldn't bear it any longer and touched her, and got an erection! I thought she would notice and get angry, but she welcomed my erection! I started masturbating next to my sister-in-law as she was sleeping. What's more, when she noticed what was going on, she secretly got horny and we ended up having an unexpected threesome!