Here is my idea for the story.
First the model of the Mom should be a bit Curved as she is a mother of 2. Second the MC should be short and hung like a horse. The sister could be a legal loli, or a sexy/bitchy older sister. And the Dad could be as secret cockold, (awaked when he sees his son do his wife).
For the story, the relation of the MC's parents are not going so well, as the dad not that sexually active as before. So they go on vacation to Vegas to try and save his marriage, but the wife is so infuriated with her husband that she would rather go in the backseat with her son and drink wine, rather than seat at the front with him.
Now, for the MC. He is at his height of his puberty, anything could make his aroused. But due to his physical condition (having a horse size dick), women his age are afraid of his "size"... In this part the MC could be a person looking for a partner or be a dom and "release" his frustration in his mother. And for the interactions, it might be cliched. Like mom teasing her son or son caressing mom while drunk or asleep.
In the timeline, there will be at least 3 pit stop, as it is a 6hrs drive. In the pitstop, the MC could meet/interact with new people or his family and/or buy things that would help seduce/arouse his mother and/or sister, or both.
As for the Ending, there could be 5.
1. Happy family - Parents reconcile
2. secret lover - mom in secret girlfriend
3. Taboo family - mom and sis loves MC and dad happily watch or joins
4. secret slave - mom and/or sis are MC's slaves without dad knowing
5. Dominated Family - MC is the Master of mom, dad, and sis.
