My step-sister, Haruki, was born when my father remarried. At first glance, I thought she was rude and awful, and that we'd never get along, but something happened that made our relationship closer. Then, when my sister found out about my girlfriend, she unexpectedly invited me, saying, "You're an introvert, but you've got a girlfriend? You have no experience anyway, so I'll be your sex practice partner if you promise not to insert it." From there, we started rubbing our genitals together, then inserted just 1cm, then just the tip, and it gradually became real, and finally we used her incredible body as a practice dummy for real sex, drowning in the pleasure of sex!