One day, while the brother and sister were alone doing nude figure drawing, the brother's girlfriend suddenly showed up. The dominant girlfriend declared she also wanted to do a nude sketch of him, and despite his hesitation, she began sucking his cock right in front of his stepsister, taunting him to "get harder." Witnessing this act, the stepsister felt a sudden surge of arousal toward her brother. Soon, sex with his girlfriend no longer satisfied him, and he began craving his stepsister's body. The two gradually gave in to their forbidden desires, embarking on an illicit incestuous relationship.