So MC is having sex with his Mom, his older sister, AND the titular younger sister. And it's revealed to the players in such a casual, matter-of-fact way. I'm *pretty sure* this should have a little bit of context added. Honestly, if Mom and older Sis don't factor into the game going forward (the roadtrip being the focus of the story), leave them out and just detail how MC and his little sis got into a sexual relationship.
The idea that MC is going to be having sexual shenanigans with other girls on this roadtrip is hampered by the fact that his main LI (Diane) is the jealous type. Players who don't like being a cheater would be better served having Diane be a cuckquean who loves to see her man fuck other girls (with or without her involvement).
Will Diane be having sex with others on this roadtrip as well?
And how the hell do you progress once you get to the sandbox?