When my sister brought her friends over to our house on a cold day, we all gathered around the kotatsu to do our homework. So, I peeked inside casually. Inside the kotatsu, it was a paradise of panty shots, with my sister's friends' panties visible up close and for free! As I pretended to sleep while enjoying the panty-viewing, my erection was discovered! I thought it was bad, but then one of my sister's friends seemed to get excited by the erection and secretly touched my erect penis, trying not to let my sister know. Moreover, she even said, "Please touch me too."