The aggravation and tension of sibling rivalry explode in the most pleasurable way.
It's so hard being the youngest one in the household, especially when picked on by the sassy, teasing older female living in the bedroom just next door. Lauren's always sticking her nose where it doesn't belong, ranting and raving, taunting and tormenting, and he wishes she never became part of the household at all. Life would be better without Lauren always in his business and giving him a hard time; he couldn't possibly learn to actually appreciate her presence or all the things his older sib could teach him. That would take some monumental event - first-time experience - his young mind can't possibly fathom... until it happens.
When his parents go out for the evening, Lauren's usual harassment begins. She invades his room, pokes around on his computer, and asks all kinds of questions about the girl at school he likes. The only thing he can count on from Lauren is her incessant invasion of his privacy and her keen ability to embarrass the heck out of him. Her questions are annoyingly personal, her lack of shyness shocks him nearly to death when she suddenly appears from behind the shower curtain all wet and fully revealed, and when she discovers just how uncomfortable and inexperienced he is when it comes to the opposite sex, Lauren goes on a full rampage, intending to teach him a thing or two.
He doesn't want to learn; Lauren can't possibly have anything good to teach him - or show him. But when she takes it upon herself to force a lesson on female upper anatomy, he can't help his bodily reaction. And Lauren, of course, knows just how to deal with that too. It may just be all a big game to her, but he is the ultimate winner.