Serious answer. It is pretty well know that most of sexual assaults gone unreported. It will put stigma on victim and put them through reliving the experience. And there're more reasons like low success rate and other that I won't bother giving references or details, but you can easily find them if you're interested.
That is even excluded the fact that the perpetrator here is both a family member and a minor. You'd easily realize why they didn't put the case.