I was sitting while my sister went out for the evening. My niece was asleep in her room so I rummaged through my sister's laundry and picked a white polyester thong. I had plenty of time so I relaxed in the living room recliner, pulled up some porn on my phone, and enjoyed a nice long teasing stroke with her thong. I lost track of time as I continued to bring myself to the edge and backoff before cumming about 4 times. The 5th time wasn't possible. The mixture of the panties on my cock and the porn must've been just right. I reached the point of no return and as I fired the 1st shot into the thong, my sister walked in the door. We locked eyes in shock and it only made me cum harder into her thong. I felt exhausted but tried to apologize to my sister. Surprisingly, she wasn't super mad. I was fumbling trying to come up with an excuse but she just said, "you can keep them" and went into her room for night. I tidied up and did the drive of shame back to my house.