I'm trying to focus on "silly, harmless porn game... don't worry about the plot," while also trying to ignore things like "If I haven't seen them since they were 12 or 13, and one is a virgin, what made them plot to invite me out with the intentions of jumping my bones, together?" and, less meta, "my cousin hugged me when I said hi and is happy I'm here. Do you think she wants to fuck me?"
It's kind of hot, it's silly fun, and it's a first attempt (compared to my ZERO attempts, so I shouldn't judge so hard), but it comes off a little odd.
A suggestion of how I'd manipulate the plot a little, to make it feel more normal?
1) With a few weeks off college, you went to that city for a vacation. Upon getting there, the event you went there to see is canceled and your hotel screwed up booking, and you have no place to stay.
Call home, talked to mom, and she suggests your uncle. Talking to him, he says yes, and informs his daughter's you'll be visiting, but tells you he'll be gone all week on a trip.
2) Cousins are happy to see you day zero. Less flirting, more excitement. After dinner, while you relax, they talk to each other and say they both used to have a crush on you, and as twins they share anyhow, and THAT is when they make their plot.
3) day one should be more... settling in and getting to know the girls. Some flirting and teasing, but nothing too far. Early in the day, they call to reserve the hot spring.
4) day 2 should be most of current build's day one
move forward from that point
That makes a better reason for him to be in town, a better reason for the cousins to decide to make a plot, and slows things down, just a little, to allow better pacing, and getting more from the girls feel more earned.