An older brother and younger sister are established on the first page or two as having a very strict household. Then their parents go away for the weekend so they relax with typically not-allowed snacks and activities. The sister changes into pajamas and they start drinking. They eventually get tipsy and the younger sister suggests watching some porn and one thing leads to another and after some dry humping they have legit sex.
Thank you for all your help!
Thank you for all your help!