DVAJ-349 A life on the brink of incest with my sister, only allowing insertion up to the first 3cm Kawakami Nanami

Unable to bear seeing her injured brother unable to masturbate, Nanami ended up helping him jerk off. Since then, secret masturbation support became a daily routine. It didn't stop at just using hands; she would give blowjobs or grind... Eventually, the two decided that "inserting just the tip 3cm in doesn't count as sex," and started having incomplete glans sex. Is inserting 3cm really safe? Knowing it's wrong but can't help going deeper, this incestuous pair is so erotic! The guilt and pleasure when fully penetrating to the deepest part of the vagina are truly extraordinary!!